A few points from the standpoint of a Turnstile Greeter.
A child ticket (Age 3-9) will, as long as it is valid for that park, unlock the turnstile and give a green light immediately. It will not activate the biometric scanner. We try to discourage children from wanting to use the scanner since it is not on and it slows down the line.
The biggest problem we have with children and their tickets really is that the child will hold on to the ticket while the reader is trying to take it away. Due to this the timing of the reader may be thrown off and the ticket will reject as "unable to read". Normally I can cure this by putting the ticket in the reader myself.
If I see any person who is having difficulty with the finger scan due to what appears to be some sort of infirmity I will almost automatically hit the necessary keys for an override. This can be a young person who appears to have something like DS or autism, or a person older than me who seems inform and not sure of themself.
If you have a child ten or older who will have a problem with the scan, just let the Greeter know,
in a nice way, that assistance is needed and usually the Greeter will just do an override to keep the line moving.
And although I do put my own ticket in the reader when I am using my
ECV to enter one of the parks, usually with a person who is not familiar with the turnstiles in a wheelchair or ECV I will just ask them for their ticket, run it through the reader, do an override and let them through the gate.