So just been shouted down for using vagina with my 5 &6 year old. The have always followed me to the toilet, so during periods, I explained quite young what a period is (blood is washing out the uterus each month as the is no baby in there. Blood then comes out of the vagina). My kid use common terms generally but if there is a problem, they use the medical names. The NSPCC say it is really important for kids to not be ashamed of their genitals (we call them genitals not privates).