They are not checking anyone's fingerprints anyway. Just your biometrics (bones sizes, lengths, etc). And kids' hands grow too fast, so they don't get checked.
My 10 yr old GS had to use the finger scan in July when we were there. When we bought the AP's we told them we needed them for 2 adults and a 10 yr old.
I think it is anyone younger then 10 that does not do the finger scan.