I tend to take my child off it, and tell them that it's someone else's turn now.

As often as not, the other parent will say something along the lines of, "oh that's fine - he can ride too" in which case I'll tell my child the other child is sharing, my child will generally say thank you (if not, I encourage him to!) and they can ride together, and I chit chat with the other parent while they do.

If the other family doesn't share, that's fine - it shows DS that not every one thinks to share, or chooses to do so, and that's life. If they do share, that's wonderful - we can all play and enjoy together, and we say thank you.