Normally, my son (2 years old) does very well sharing with other kids. In fact, he's remarkably social, even with children he's never met before.
And therein lies the problem.
On a recent trip to the museum, he was very excited by everything that was going on, but mostly by what the other kids were doing. The problem is that he wanted to play with other kids, including older ones, but not everyone takes kindly to a 2 year old running over and "helping".
I definitely don't want to discourage him from the idea that sharing toys is the best way, but how do we address him running over to strangers and expecting to share? In some cases, that's appropriate, but when an older child is carefully arranging velcro stars into specific constellation patterns, my son running over and moving stars around or slapping new ones over the other child's isn't appropriate. Similarly, if there's a bin of blocks or Legos, my son wants to contribute to the other kids' projects, even though there's enough space/materials for him to play on his own.
Some kids are going to be alright with this, but others may not be.
How do I approach this with my son? Ideally, I'd like him to ask the other kids if he can play with them, but at this age not all of the kids he wants to play with are verbal enough to respond appropriately, plus he moves so fast that its difficult for us to anticipate where he is going, in order to get him to ask first.
What if the child he asks says "no"? When we taught him how to share, we tried to be clear that he doesn't have to share, but he should. This may be a little harder to grok coming from the other direction, particularly if the toys and designated play areas clearly are communal. Its much easier to say "this other little boy/girl is playing with that now; let's find something else to play with" than "this other little boy/girl is playing in this area; you can play too, but you have to stay over here and leave them alone."