If the lump/bony bump is indeed a permanent fixture near the base of the skull, it's most likely the classic "External Occipital Protuberance". If, however, this is a new growth on the skull, or if...
That depends on how it got there, if the child hit his/her head and it's just a small bump with no headache or vision problems it is probably OK. You should check with your pediatrician anyway just...