If the kid is 10 and younger then they're free. Just use the wide gate with you (their more gates than turnstiles, and there's always a wide one for handicap that delays shutting as fast) . Parents with kids use it. If they're a teen you go go through the application process to get them discounted cards (if you want to take the time, to save a bit).

One thing that confuses newcomers using the tap function on their cards or phones is that you can't switch "devices/card". If you start your trip scanning in with your physical card, then you need to end your trip scanning out with that same physical card, not your phone's apple wallet that has that card number. It treats them as separate devices. So if all your family has smart phones, then you can use the same card number on all different devices. you just can't scan and then hand your card/phone back to be used by the next person. Everyone needs their own device/card