All my friends with children rave about Kidzania as the best thing since sliced bread. I've no idea if they're bigging things up too much though. May will be pretty hot (mid to upper 30s) so be prepared...
Actually, if I were you I'd hire a 4x4 for a day and drive over to Dibba or Fujeirah where it's a few degrees cooler, the mountain scenery is beautiful and the sea has turtles.