Sign in waiting room:

I'm reminded of a sign I saw in a coffee shop: "Unattended Children will Be Given a Free Kitten and an Espresso"

Leave the kids at home. Get a sitter.
Kids don't belong everywhere.

I've seen the video, and everyone is at fault here..the art gallery for not roping off such an expensive art work, having a guard, or at least a warning sign.

The mother, for sitting there reading obliviously while her two little boys chased each other through the art works.

The two little boys are the least at fault. They were left to their own devices, and boys almost always run and roughhouse when playing. They should have been warned, and kept close to a parent, but nobody said anything to them.

Until the kid turns 8 they are legally completely innocent, the parents are on the hook for any actions of the child, similar to if your dog was running loose in the gallery.

In case someone needs to look at it a different way, what if one of the parents knocked it over? Are they responsible? Until the kid reaches the age of adulthood parents are responsible. The kids are an extension of the parents.

Parents should be fully responsible for their kids. They made no effort to prevent their children from knocking it over.