Detsky Dom issues that need attention

Lately Kristina has been playing that she is a baby and "the kids" take her hot water, in a sippy cup. She pretends to cry like a baby and Mama (me) has to get it back for her and protect her while she drinks it. I am going along with the game. I have no doubts that this happened all the time when she was young and it really annoyed her. By the talk and actions it seems like she goes back to when she was around 1 or 2. It gives her great comfort to know that now Mama is here to make sure she finishes her water without anyone taking it away from her. I have read that this is all part of the healing process. She is making up for those lost years without Mama there. When do I say OK, you are old enough now and just tell the kids is your water and you are still thirsty? Not sure that water is the best to use as a sharing exercise.