Vagina, vulva, penis/ wiener.
Also 'privates'.
I've heard boys aged 6-8 refer to nuts and gonads/nads.
'Butt' covers the entire area.
'Butthole' refers to the anus.
Ass isn't a term used by elementary school kids, in my observation.
It's a term with several meanings.
My kids grew up in the 90s and 00s in the US, and I do a good deal of work with kids.
In general I notice far less euphemistic speech around genitals in the area we live, and in my work, than in the UK, and little or no jokey banter centered around genitals. Its just not part of the culture, especially in middle class areas.
Your child won't need to worry that conversations will revolve aaround body parts or bodily functions, and nobody will blush if correct terms are used. (This may be geographically dependent, however - I do not have experience of the South).
Poop is the term usually used for poo.
Pee is used for wee.
Toilet is the term for loo.