Do an 8 year old need an ID to fly in US?

We fly fairly often with our two kids. While we do have passports for them, we've never been asked for them. Children are not required to have ID when flying within the US. Interestingly, the security folks always ask our older son for his name and then ask him to tell them our baby's name. Our son goes by a nickname that does not match his legal name (which is printed on his ticket). They hold us there until we explain to him that he needs to say his "real name" and only when he does so do they let us go on. It's pretty clear they're doing whatever they can to make sure the kids you're traveling with are the ones you bought a ticket for.