Take Turn Timer App for iOS
'Take Turn Timer is your solution for teaching your kids how to share. Each person can get an equal turn and Take Turn Timer will let them know when the turn is over even when they're off playing in other apps.
It's even useful with one child. As one user put it: "is your kid dominating your iOS device? Reclaim some time for yourself by teaching your kid to share"'
This app cost $0.99