Shouldn't get a bad credit rating, just don't use the sim if you do cancel the direct debit, they can't say you didn't pay for something you used then!
I see your doing what I done at 16, was a pain cos mum always had to be there when I wanted something doing of to upgrade. Now 18 so got a new contract in my own name.
oh well it's worth a try, at least i have an idea of what to say, cheers!
and vodafone are usually alright, i just explain and they don't ask for her