Arguing with kids is rarely productive. They have yet to learn actual logic.

If something as big as bigfoot was actually living in the US then someone would at least have found a dead one by now. There would need to be a few of them to be viable. I get that people want to believe in something mysterious. There are 'Nessie' hunters in Scotland still looking for a giant creature in a lake. I do think there are creatures yet to be discovered, but they are likely to be small.

Have fun with your events.