As long you have instructed them in:-
The correct use of, and polite behaviour on, public transport
Not blaring out crap too loud music on their mobiles
Not swearing loudly in front of me
Not discussing sexual acts that I have never thought of in a loud voice especially if you partook of them on the climbing frame in my local park or on the bench where I sat this morning!
Accepting they are not the centre of the
Universe to anyone other than their
parents/family
I live near a FE college and many of the YP are lost souls when they start so I do think a couple of trips on the bus/train with parents/elders is in order. Otherwise they get on and freeze if the bus isn't empty and the might have to sit next to a stranger, an older person, a parent with a child.
I sound as though I hate YP but truly I like many of them but feel they must be socialised before being let loose on an unsuspecting public!!
I realise OP's child is just 12 so I'm sure much of this doesn't apply. I just think that we have lost the village way of child-rearing, in which any misdemeanours were known by parents before the children got home and when it was possible to remonstrate with children without being accused of assault.