It takes about an hour to hike from the town to the Cirque if you go straight without stopping, but it's lovely outdoor scenery and you'll want to stop and play along the way. Plan to spend the whole day at Gavarnie.
From the town of Gavarnie, you can hike to the Cirque, or kids can ride on a gentle donkey or mule, led by parents. "Rent" donkeys in town to go and come back. The hike is not difficult, it's a wide gravel path (plenty wide for strollers), and along the way you'll see yellow buttercups, blue irises, and little streams for kids to play in.
When you reach the Cirque, you'll have to leave the donkeys. There's a hotel-restaurant with outdoor terrace and kid-friendly food. If you want to picnic on the rocks, bring your lunch along; you can get everything you need for picnic in town before setting out.
Hike up the rock basin to the base of the big waterfall, plus you'll also get a good look at other waterfalls cascading down the rock faces. Tip: If you hike to the waterfall, wear sturdy shoes, the trail here is rocky.