Well the bike situation has happened to me but my ds took it but I jumped up and went to get him off but the parents said he was ok to play with it as it hasn't been used, I once had two kids picking on him at the park and I saw one push him so I marched up there and said in a firm voice don not push my son and they left him alone after that, if the parent had issues with that I would tell them if they were parenting them I wouldn't have to.

My kids rarely wear socks at the play centre because they always take them off, and we go with my little sister and my kids so often have the big kids 10-5 in the toddler room though they don't play rough