Younger kids can run wild at Galipy. They'll sleep soundly after hours of climbing, sliding and swinging on the equipment.
You're spoiled for sightseeing choice when staying in Orvault. Galipy is a good place to kick off here, but there are loads of other worthwhile spots to appreciate. Chateau of the Dukes of Brittany, Zenith de Nantes, and Jules Verne Museum are near the outdoor play area and and will cap off your day nicely.