I agree that I would say something. The first situation is IMO very rude. Because if the child is resistant to getting off you have little choice but to hunt down the parent (you can't pull the kid off) or just hold the bike till they get off. I feel that in small familiar groups (e.g. mothers groups) then if you bring a toy then be prepared to share but in the park then leave alone toys from people you don't know.
In the play centre situation we always say something directly (nice but firm) to the kids. They are usually apologetic and leave the area. If they didn't I would say something to management because it is a safety issue not an etiquette issue. But we have had the difficult situation where an older boy was babysitting his younger brother with his parents nowhere in sight (they weren't even in the centre) and they were really too rough for the young section but too young for the older section. Ultimately it is up to the parents to supervise especially in the bigger playcentres where you can't see them well from the tables and I get really annoyed at kids asking me to open gates to get them in or out of sections when I don't know whether I am going to get abused by the parents for letting them in or not letting them out (I have had both).