I used to be the type to jump in and ask my kid to share, but lately I've been keeping some distance to see how my kids react. Some times they offer to share right away, some times they need to be nudged to give the other kid a turn. It's interesting to see them learn to negotiate their little worlds.
At age 3 my daughter encountered a bully who demanded a toy that she had recieved for her birthday. I observed at first to see what she would do. She usually did not have a problem with sharing, but I think she sensed this was a different situation. The mother of the child came up to her and pointed out that her son was very upset that she didnt give up her toy. When my daughter again refused, the mother turned to me and pointed out how sad her child was. I asked my daughter if she wanted to let him borrow it. She replied "no." I turned to the mother and said, 'She said "no".' As much as we want our kids to be kind and charitable, we also need them to have a backbone.
So glad you raised this point. As important as it is to teach our kids to share and be generous, we also need to help them learn to say “no” when they feel uncomfortable or like someone might be taking advantage of them.