I agree with most people in this thread.
for example, in a downtown park in my city right next to my school there is a tank. it's not a monument or a memorial, just an old tank with a flag next to it. I think it's dedicated to a group of people or something, but it's completely set up for something like this. kids climb on it every day, there is no fence around it, and it never gets damaged anyways. it even used to be open and you could sit in it, but in the early 90s a kid suffocated inside it so they cemented the hatch closed.
if this cannon that stept was hitting is anything like the tank then i'd say it's completely fair game.