If you have young children (or children that act like toddlers and preschoolers), you may have noticed they sometimes get upset when you want them to stop doing what they're doing.  I would suggest you give them a choice about how long they can play before they begin to play that will end their playtime.

"Honey, Do you want to play for 30 minutes or 45 minutes?"

"Do you want a 3 minute or 5 minute warning about getting ready to end your play so we can do sooemthing else?"

"When your time is finally up, are you going to come quick and snappy or will you show me turtle behavior and move really, really slow?"

If your children give you answers to these questions, they have a vested interest in following through.  Right beofre the time is over, ask the last question again and then see what happens.