I would have phoned the papers as soon as I got chucked off the bus!!!
This used to piss me off when DS was younger.
Just after he turned three, he got glasses, and for some strange reason knob-head bus drivers thought this made him over 5! Even though he was still in a buggy!!!
It got so bad, that I had to get him to take his glasses off until we got on the bus, just to stop arsehole drivers thinking im some sort of fare fiddler!!!
I however never encountered what you have, I think it's disgraceful the way you were treated, and I cant believe the fuzz never took your word regarding your DD age. Could they not have radioed the station and done a PNC, surely that would have shown your DD age?
YADNBU, and let them waste tax payers money, taking you to court, and make sure you phone your local paper, The Sun, DM!, the lot! first thing in the morning!