I hope there was some kind of notice to treat the first statement as true (even if you believe it to be false in reality) for purposes of answering the question after the second statement. Granted I'm not sure the younger child could understand such a notice, but maybe there's a more elegant way to get those instructions across.
I was actually kind of hoping when I saw this experiment start that it was going to have a card saying that glass broke and asking what caused it to break.