The kid's daypack is a budget, and water spends half of it

Quick answer: Keep a child's loaded pack to roughly 10 percent of body weight, up to about 15 percent for an older, fit kid. Weigh it on a bathroom scale before you leave the driveway. Let the kid carry their own water, snack, whistle and layer. You carry the safety gear.

Do the math before you buy the cute pack

A common rule of thumb for kids on day hikes is a loaded pack of about 10 percent of body weight. A strong, experienced ten-year-old can stretch toward 15. A kindergartner should stay under 10.

Take a 40-pound five-year-old. Ten percent is four pounds, and that's the whole thing: pack, water, food, everything.

A one-liter bottle of water weighs about 2.2 pounds. Fill it and you've spent more than half of that kid's allowance before a single snack goes in. Add a cartoon-character pack with a stiff plastic back panel, and you're over.

So do the math with a scale:

• Weigh the empty pack. A good kid's daypack runs around a pound to a pound and a half. Novelty packs often weigh more and carry worse.
• Subtract it from the budget.
• Fill the bottle and weigh that too.
• What's left is the room for everything else.

For our 40-pounder, that's about four pounds minus a pound of pack, minus half a liter of water at 1.1 pounds. You've got roughly two pounds for a rain shell, snack and whistle. It's enough. It's just not enough for the rock collection.

The pack itself

Fit beats features. Check three things.

Torso length, not age. A pack made for an adult hangs off a small back and drags the shoulders backward. Kid packs come sized by torso for a reason. The top of the pack shouldn't rise above the ears, and the bottom shouldn't sit below the hips.

A sternum strap. It keeps the shoulder straps from sliding off narrow shoulders, and it's the best place to clip a whistle. Cheap and worth it.

A hip belt, for heavier loads. On a pack carrying more than a couple of pounds, a padded hip belt moves weight off the shoulders. Snug it so it rides on the hip bones, not the stomach. Under two pounds, a simple strap is fine.

Skip the giant main compartment. A kid with a cavernous pack fills it, and they fill it with stuff you'll be carrying by mile two.

Water is the whole job

Kids don't drink on a schedule. They drink when you make them, and they're often behind before anyone notices. So build drinking into the hike. Stop at every landmark, every bridge, every creek crossing, and say "sips." Don't wait for a complaint.

Bottle or bladder? Both work, and both have problems.

• Bottle. The kid can see how much is left and open it without help. But taking it out of the side pocket means stopping and fumbling, and small kids drop it. Pick a wide-mouth bottle with a loop or a lid they can twist off alone. Test that at home.
• Hydration bladder with a hose. It's easy for the kid to sip while walking, which means they drink more. The trade-off is that kids chew bite valves, leaks soak the pack, and nobody can tell how much is left. If you use one, size it at half a liter or one liter and check the valve before you leave.

Carry the bulk of the water yourself. A liter for the kid and an extra two liters in your pack is plenty for a short, cool hike. Go heavier when it's hot or the trail is exposed. You'll drink some of it, and when the kid's bottle runs dry, you've got more.

Pack a salty snack with the water. Pretzels and crackers do the work and kids actually eat them.

Gear that earns its weight

Here's the kid's pack for a typical half-day hike, with the weight in mind:

• Water. Half a liter to a liter, depending on size.
• A snack they picked. If they chose it, they'll eat it. Dried fruit, a sandwich bag of crackers, a granola bar. Nothing that needs a spoon.
• A rain shell or light jacket. Weather turns. Cold and wet is how a fun hike becomes a miserable one in thirty minutes.
• A hat. One they'll wear. A cheap baseball cap beats a fancy one that stays in the pack.
• A whistle on the sternum strap. Three blasts means "I need help." Teach it in the backyard, and make them blow it loud once so it's not scary later.
• A small trash bag. Every kid packs out a wrapper. It's the first lesson in leaving a place better than you found it, and it takes ten seconds to teach.
• One job item. A notebook and pencil, a cheap compass, a bandana, a magnifying glass. Kids carry more happily when they have a job. A kid who's the "navigator" will walk a mile farther than a kid who's just along.

What you carry

The safety gear isn't for the child's pack. It's yours.

• Water beyond the kid's bottle.
• A first aid kit with blister care, which every kid needs eventually.
• A map and a charged phone.
• An extra layer for the kid.
• A headlamp, even for a daytime hike. Trails take longer with short legs.
• Sun protection.

If you haven't built out a full kit, the ten essentials are the place to start. Scale them to your group.

On sunscreen: wear something every day. Any broad-spectrum SPF 30 or higher beats skipping it, and sun damage is a proven cause of skin cancer. I'd choose a mineral formula with zinc oxide or titanium dioxide. Those are the only two filters the FDA has proposed as generally recognized as safe and effective, and mineral is gentler on kids' sensitive skin. The trade-off is a white cast, which a kid will complain about for about four minutes. Thinksport SPF 50 (Thinkbaby for the little ones), Blue Lizard Sensitive Mineral SPF 30 and CeraVe Hydrating Mineral Sunscreen SPF 30 are all worth a look. Reapply every two hours and after swimming or sweating.

The rules to set at the trailhead

Say them out loud, once, before you start.

Stay where you can see me. On a trail with turns, that means in sight or within earshot. Little kids stay behind you or in front, never out of view.

If you get lost, stay put. Stand still, blow the whistle in threes, and wait. Hug a tree if that helps. A child who keeps walking is a child who's harder to find. Practice this once at home so it isn't the first time they hear it.

Rocks stay where they are. This is the pack-weight rule in disguise. Every kid wants a souvenir, and by mile one a pack can hold four pounds of granite. In many parks and public lands, collecting is prohibited anyway. Take a picture and leave it. If you want a keepsake, one pocket-sized item goes home, and only where it's allowed.

Check the sky before you leave and again at the first break. Weather is the thing that turns an easy morning into a hard afternoon, and reading it in time is a skill your kids will pick up by watching you do it. And if this is your first outing, keep it short. A one-night trip is a better start than three days, and your first family camping trip should be just that.

Weigh it, then let go

Put the loaded pack on the scale. Under budget? Good. Over? Pull something out, and it's usually the extra bottle or the third snack.

Then let the kid carry it. Don't take it from them at the first whine. A pack on a child's back is a statement: you're part of this, and you're pulling your weight. Even if the weight is two pounds.

Cinch the straps, check the whistle, say "sips" at the first bridge. That's the whole system, and a kid who's carried his own water up a hill will want to do it again.