I guess it depends on the kid. If the 12 yo clicked with my child(ren) and seemed capable, yeah, in a heartbeat. My best mother's helper is my 11 yo dd. She just has a gift with babies and little ones. She is soft spoken, patient, and just cues into what is going on, etc. Her OLDER sis is more gifted in other areas.
So yeah, ... that is my take on it. I don't know what the going rate is for compensation though.