I watch to see if they can resolve the toy conflict, but when they don't/won't/can't I gently remind them that all the toys belong to Mimi and I am sharing them with you. So please share my toys with each other. This puts the toys in a different perspective and I find it works. I will often hear if there is a toy sharing conflict, the older children reminding the others "these are Mimi's toys, not yours"