Our puppy has a habit of humping a cuddly bunny that he especially likes humping after his evening meal! I think it is a sign of dominance and shouldn't be encouraged although apparently males and females are known for humping. Perhaps when she does it your son should say 'no' and refuse to play with her for a few minutes. It can cause problems when the pet is adult. I'm not sure what to do about our pups humping behaviour as my partner and I thought it funny when he started doing it and still find it amusing - we could be setting ourselves up for problems later! Are you planning to neuter your pup? I think the behaviour settles down when they are neutured/castrated.