^ OP,
Yes, sorry, should have reassured you that particular book's aimed at ages 5/ 6ish and older.
I suspect the author/ illustrator (Babette Cole) included the...um... adventurous "fitting together" pictures more to lighten the mood for the adults, whilst they broached what can be a tricky subject for some!
(space hoppers, anyone?).
I grew up with medics in the family, so factual terms were our norm: we never had "the talk", as such. Just ongoing learning about which things do what.
TBH, there shouldn't be any reason to feel awkward when biological terms such as "vagina", "vulva", "penis" etc. are parroted by a small child in an innocent I Know How Babies Are Made-type way to lucky old Jean on the checkout at Sainsbury's (though I understand maybe Jean might feel otherwise! And parents, too; in the end, it comes down to whatever vocab you and your family are most comfortable with).
Nursery age, when baby bro was on his way:
According to her, curious questions from me were simply answered that every bit of the body had its own important job to do.
"Daddy gave me a tiny seed ('some people call it sperm') which mixed with a tiny egg ('some people call it an ovum') from me.
This makes the baby, who's growing bigger in a special place called a womb (more likely uterus, knowing my Mum!), not my stomach/ tummy".