me and my sister used to call ours willies as well, don't think we realised it was gender specific.
we would also just say our ''we'' or ''wee''
we used to say boffed, trumped or pluted, for farted, still do actually...
a child saying ''passed wind'' is more like to be skitted than one who says ''pumped''
tuppy comes from tuppence probably, and i believe was the origin of ''tipping the velvet'', it used to be ''tupping the velvet'', (cunnilingus)
you could try ''front bottom''...lol