I would complain to CS and report that flight attendant. I wonder if that flight attendant would have reacted the same way if you had been a deadheading pilot or an FAM?

If I had been on paid ticket I would have taken a seat in economy, waited until the door was about to close then gotten off. So either the flight gets delayed while they redo the paperwork or they fix the issue. It can't be that hard to physically remove a child.