The generally used figures is that 1 in 10 children are abused with girls being at around 2/3 higher risk than boy. Child Sexual Abuse is really not anywhere near as rare as people believe. Some figures go as high as 1 in 4 children but they are definately paranoid, 1 in 10 is fairly well accepted.
The most likely person to sexually abuse a child in the UK is his/her natural father, followed by the step-father (this fluctuates though depending on the last stats count, it often swaps places with step-father first). I forget the rest of the order of risk but mother comes fairly high up on that list. Strangers account for a tiny fraction of sexual abuse incidents (or at least, of the reported ones) and then those incidents tend to be isolated incidents whereas abuse by trusted people like the babysitter or the big brother tend to not be an isolated incident.
Of the abuse incidents that happen almost none of the children abused will find the courage to tell on thier abuser before they reach adulthood. Agencies believe that kids who are known survivors of abuse before they are 18 account for under 10% of the kids living with it.
So paedophiles are certainly numerous (and/or very busy) but they are unlikely to be standing round the edges of the park in grubby macks where you can easily spot them. They're those nice normal happy men and women you greet at the school gates and send your kids to sleepovers with (don't worry, most paedophles wouldn't risk it to rape a child at a sleepover, thier own child is far less hazardous).
You can't spot a paedophile. Ok so some of the rather dim ones are a dead giveaway, but those are the ones that get caught early on and taken out of society. The ones that are any good at it (for want of a better expression) will work thier way through many victims. So the number of paedophiles is lower than the number of victims, but how much lower is not something I can quantify.
Paedophiles are all over the place, although not neccessarily in the form we have been conditioned by the media to fear.