Do you book one of them as an adult when booking hotels?
My eldest is 13 - almost all hotels only allow 2 children per room, but would allow 2 adults plus 2 children.
13 year old is as mature as some adults and adult size... She doesn't particularly look like an adult though, she doesn't look especially young either, she just looks her age.
Would you book her as an adult? I have no problem paying adult price for her at a budget to lower mid range hotel.
This must be a very common thing. It's very annoying.