Thanks all. I googled last night & freaked myself out (yeah, I know)... Dr Sears website mentioned that preoccupation with it might mean behavioural problems
I woke up at night thinking of her other issues - she finds it hard to play by herself for any length of time, really.
She does use the ipad a lot & watch tv because she is very "intense" - very active, a climber, a daredevil... All these things make me wonder if I have failed her. I don't always know how to handle her & I really hope this fixation isn't
linked to some kind of psychological problem

Becsuse the article mentioned that if they withdraw from people in order to masturbate, that is worrying... She does that at my mum's house - excited about going to her nana, but when she gets there, she gets down on the floor & starts....