Firstly, try not to get involved – unless they are at each others throats, or they ask you to mediate. They may think you are picking sides.
Secondly, there was recently an article in the Hamodia about forcing children to share. The crux of the article was, that if a child has a toy and another wants it, we should not tell the owner to share, as this gives the child a message that he is not the owner, and he will get upset and resent the act of sharing. He must be explained that he IS the owner, and if he wants to do the right thing / he wants to do a mitzva, he should share.
In theory it sounds a sensible point.
Anyone disagree?
Maybe we could make a new thread for this discussion