I have 3 DCs. DC1 knew "no" from 3yrs old and stood by my side when required (now almost 8). DC2 is like the tasmanian devil, he is currently almost 6 and even when we are having to physically restrain him, he can fight with the strength of an adult. DC3 is only 3 but knew "no" from the age of about 12mths.
As a general rule, I don't judge parents unless they casually just stand there if something happens. I am on my DC2 like a hawk because I know he is capable of doing some damage, even though he full well knows what "no" means, he choses to ignore it and there is nothing we can do about it. Doesn't mean we ignore it though!!
Although, despite the parents needing to take responsibility for their DC, the venue and the sculptor in this case also need to take responsibility to ensure that if someone did knock into the sculpture, it would withstand the force.