Sharing is, quite understandably, a big issue in my house. I have a 2.5 year old of my own and look after (at various points) two 18 month olds, a nearly 3 year old and a 4 year old. When I started out a year ago my daughter had 2 special objects/toys that I didn't make her share, everything else was shared with all the children who came. Fast forward a year and she's older and has more of her own possessions that she remembers being given - and she doesn't want to share them.
How do I work this so it's fair? I have loads and loads of toys that my daughter doesn't see as hers and that are shared with everyone equally. But there are now half a dozen favourite toys that were specifically given to my daughter that she doesn't want the other mindees to play with. I think she's too little to be expected to exclusively play with these upstairs away from everyone else but playing with them around the other mindees causes major ructions! It seems mean to make her share her own things but also mean to expect very young children to watch her play with something they want and aren't allowed. And, of course, once they work out it's special to her they want it all the more!
I have older children too but they have all their special toys in their room as they're too old for the minded kids and as they're in school all day it's less of a problem anyway.