With the park situation, I would be so shocked that a child the age that you mentioned isn't so closely monitored that they are able to approach a stranger and get the bike in the 1st place.. Then after that shock wore off, I would then ask for the bike back as DS (or in my case DD) wasn't asked if you could use it. (If I was asked, I would most likely say yes if DD was otherwise occupied)..

For the playcentre, I tend to shield DD if there are rough kids on there that shouldn't be. And explain to her that they shouldn't be there, but she will get her go soon. I also don't expect older kids to make allowances for her if she is on their age group things. But while at a local shopping centre, my mum has asked bigger kids to leave the equipment. Wasn't only a safety issue for the younger kids wanting to use it, but they are designed for certain age/weight ranges, and these ones were definately not close to the age group specified.