Quite frankly: that line should be parents to draw and enforce, within reason. The liability should not be on the dating sites to prevent it from happening, not should they be forgiven for ignoring it when it does happen and it's brought to their attention.

I'm in the unenviable situation of having had one of my big projects shut down for precisely this reason: it's too expensive to moderate sites that kids may use. The reason for that is actually very simple, proved by data, and verboten to say.