Yeah, a child that age 'should' understand how to share but if they don't know then it isn't really their fault, they might not have had anybody role modeling it for them so how are they to know? Or they could know but are tired, cranky, in need of attention; kids are all still learning and will never be perfect all of the time so I would try to interact in a positive way with the other child, something like "Oh DD1 it looks like this boy/girl isn't quite sure how sharing works, how about we show them and help them understand, they can have a turn first and then you can have a turn or maybe there is something we can play together?". Compassion and understanding can go a long way. I wouldn't snatch it back off the child, that is a confusing message imo. If that didn't work I would offer something different to do or take my DD1 elsewhere like you did, explaining that what the other child did isn't okay but they are probably still learning or having a rough day and let DD1 know I was proud of how she shares.