i think it's important that children learn to share but not feel as though they can't say no occasionally.
A friend flat out refuses to get her child to share his toys when we're there and we have to take toys for our dd to play with
Another friend today took a ball to the park for her dc but wouldn't let any of the other children in our group play with it, when the ball was kicked to another friends dd as the dd tried to kick it back the mum shouted "no no that's dcs ball he doesn't share"
What's the right lesson?