I've known a few
Philip people, and they've all answered to
Phil. And all the
Philippa people have been
Pippa. So a boy
Pip would surprise me, Great Expectations or not!
I wouldn't expect a body-part issue with
Amos: when kids are at the stage of finding body parts comical, they tend not to use the anatomical term:
Willy would be much more likely to attract their attention. What I would expect would be shortening
Amos to
Amy ... that would give naughty kids a lot of pleasure.