Actually it's perfectly normal to have "unwanted thoughts" at varying frequencies. Although some people may not admit to it for various reasons, everybody experiences them sometimes (e.g. imagining harming a baby, which you rationally would never even consider).

The difference is that sane people don't act on these thoughts and notice them as unwanted (and seek out psychiatric help if they impact their quality of life).

I would imagine it's not any different for pedophiles, except they additionally suffer from an inherently dysfunctional sex life with all the psychological consequences that can have (the one thing that turns them on is the thing they try not to think about).