Are small children required to have a ticket?

Whenever there's doubt call The Source. Google the phone number and call., That's the best response.

The reality of the situation is no. I am sure if you had a couple of kids without tickets with adults who did, they would give you a quick "next time" speech and let you in. However, since the tickets are 100% government subsidized, err on the side of one extra than one too few. It also allows you to include someone else as a last minute surprise.

Yes

Call to ask how old.

My understanding, if they are walking then yes.