LO, age two, is effectively an only child (his elder brother was stillborn, his elder sister died neonatally) so although he isn't our only child, sadly he is being raised as one.
And he's very good at playing games and joining in. He has more confidence at two than I had (ever, even now) and will just walk up to adults and children alike and start chatting or playing, he's happy to join in with whatever game is going on.
Mostly the other children are happy to play and let him join in. We met two young brothers at the park the other day and the eldest, who was six, took great care of LO, helped him on the slide etc. The younger brother, who was only just four, was delighted to be bigger and older than someone for a change and so was really happy to have LO to play with him too.
And today we met a little girl, about the same age as LO who seemed to be an only child (both parents with her but no siblings in sight) and she was happy to run around and play too.
The other only children we know usually seem happy to play and the only issues with not wanting to share are the usual ones associated with toddlers who are still learning what that means. Sometimes they will share happily, sometimes it's total meltdown.