Start with Zion in late April, not Yellowstone in July

Your first national park with kids should be one you can walk across in a morning. That rules out most of the famous ones.

Zion is 229 square miles and you'll spend 90 percent of your time in a canyon six miles long with a free shuttle running up and down it every seven minutes. That's the shape you want. Park the car once, ride, get out, walk a paved trail along a river, ride back. Nobody in the back seat asks how much longer, because there's no back seat.

Now compare Yellowstone. It's about 2.2 million acres, the Grand Loop Road is 142 miles, and a single bison standing in the road can eat 40 minutes of your day. It's a magnificent park. It's a terrible first park for a four-year-old.

So pick by geometry first and scenery second. Then pick your week.

The three that forgive beginners

Zion, Utah. Go late April to mid-May, or October.

Springdale sits right against the south entrance, which means real food, real beds and a town shuttle that drops you at the pedestrian gate. Stay there, not in St. George. The Pa'rus Trail is 3.5 miles round trip, paved, flat, and takes a stroller or a kid on a balance bike. Riverside Walk is 2.2 miles round trip, also paved, and ends where the canyon narrows to a slot. Lower Emerald Pool is short and shaded.

The annoying part: by 8:30 a.m. in spring the shuttle line at the visitor center can be half an hour long, and the parking lot fills earlier than that. Sleep in Springdale, take the town shuttle to the walk-in entrance, and you skip the whole mess.

Skip Angels Landing. It requires a permit lottery and it's a chain-assisted ridge with long drops. There's no version of that hike that belongs on a trip with a seven-year-old, and the rest of the canyon is better anyway.

Summer highs in Zion regularly run past 100°F. The river is the only relief and it's crowded. April and October are the answer.

Great Smoky Mountains, Tennessee and North Carolina. Go the first two weeks of May, or the last week of September.

No entrance fee. You do need a parking tag if you leave the car for more than 15 minutes, which runs a few dollars a day or around $40 for the year, and rangers check. Buy it online before you leave home.

Cades Cove is the single best thing in the park for kids: an 11-mile one-way loop through open meadow with log cabins, a working grist mill and a genuine chance of seeing a black bear from the car. It closes to vehicles on Wednesdays in summer so people can bike it, which is a gift if your kids ride. Laurel Falls is paved and popular, roughly 2.6 miles round trip, and it's the trail everybody does for a reason.

The honest downside: the Smokies are a green tunnel. There's no single jaw-dropping overlook that makes a kid gasp. Kuwohi, the peak long labeled Clingmans Dome, comes closest, and its road is closed December through March. Gatlinburg outside the gate is loud and full of pancake houses and mini golf, which your kids will love more than the park and you'll tolerate for one evening.

Acadia, Maine. Go mid-June, or the two weeks after Labor Day.

Small, coastal, and the closest thing America has to a park you can do by bicycle. The carriage roads are crushed gravel, closed to cars, and gently graded. Rent bikes in Bar Harbor. The Jordan Pond path is flat on the near side and the Jordan Pond House serves popovers, which is a bribe that has worked on children for a century.

Cadillac Mountain summit road needs a timed vehicle reservation from late spring through October. It sells out. Some slots release months ahead and some release only a couple of days out, so set a reminder rather than assuming you'll walk up to a booth and buy one.

The ones to save

Yellowstone. Worth every mile, but budget five nights and base in two places, not one. Geysers and wolves hold attention better than any view in the country. Just know you're signing up for real driving, and July is the crush.

Yosemite. Valley traffic in summer is a parking lot with waterfalls. Go in late May when the falls are loud and the Tioga Road hasn't even opened yet, or don't go until the kids can hike.

Glacier. Short season, vehicle reservations on Going-to-the-Sun Road, and a lot of high exposure. Spectacular, and not a beginner move.

Arches and Canyonlands in June. Slickrock at 100°F with no shade is how a family learns to hate hiking. Go in late March or November instead, and check whether timed entry is running.

Grand Canyon sits in between. The South Rim is easy: paved rim trail, free shuttles, and a hole in the ground that shuts a kid up for a full minute. But it's a one-view park for small children, and the hike down isn't optional fun, it's a commitment with a 4,000-foot climb back out. Two nights, tops, and pair it with something else.

The calendar rules that actually matter

Shoulder weeks beat shoulder months. The last week of April, the first week of May, and the two weeks right after Labor Day are the sweet spots almost everywhere in the lower 48. Schools are in session, lodging drops, and rangers have time to talk to your kid.

Avoid the week of July 4 in any park you've heard of. Avoid the second week of October in the Smokies and Shenandoah unless you enjoy leaf-season bumper-to-bumper. And know that Utah's spring break crunch is March, not June.

If you have to travel in summer, go early. Trailheads in Zion, Acadia and Rocky Mountain fill by 8 a.m. and empty out again after 4 p.m. The late-afternoon hike is underrated, cooler, and the light is better.

The reservation layer nobody warns you about

Three separate things can require booking, and people usually discover the third one in the parking lot.

• Entrance. Timed-entry permits now run seasonally at several parks, including Rocky Mountain and Arches, with a second permit for Rocky's Bear Lake corridor. All on recreation.gov.
• Lodging inside the park. Rooms at the historic lodges open roughly 13 months out and the good ones go in hours. Campgrounds open six months out at 10 a.m. Eastern.
• Specific attractions. Cadillac summit, cave tours at Mammoth Cave, the Elkmont firefly lottery in June. Separate bookings, separate release dates.

Buy the America the Beautiful annual pass for $80 if you're hitting three or more parks. If you have a kid in fourth grade, look up Every Kid Outdoors first, because that pass is free and covers the whole carload.

Make the kid an employee

At the first visitor center, ask for the Junior Ranger booklet. It's free, it's a few pages of puzzles and a scavenger hunt, and a ranger swears the kid in and pins a wooden badge on them at the end. My honest read: the badge does more for trip morale than any hike on the itinerary. Kids who are collecting something walk farther and complain less.

Give the oldest one the park map and let them call the next stop. Let the youngest carry the binoculars. You'll trade some efficiency for it, and you'll get a kid who thinks the trip was partly theirs.

Book Zion for the last week of April. Do it before the shuttle schedule goes up and the good rooms in Springdale are gone.