Such as just now:
Dd(9) has been playing with ds1's (11) Action Man in games with her Bratz. He hasn't played with it for years and years.
However ds2 just bought an Action man at a car boot sale and ds1 suddenly decides that he wants his action man back so he can play with ds2, even though dd is in the middle of a game.
Ds1 says it is his so he deserves it back, and dd says she is playing with it so she deserves to carry on.
What would you do?