I think that my little guy is too young to be worried about semantics. We use every opportunity we get to discuss how we all became a family. We always speak respectfully of his birth/first/ other/ etc. family.
His birthmom & birthfather don't mind this terminology
The book (Tummy Mummy) , I feel, is very age appropriate for little ones. My preschooler understands he grew in another woman's stomach. When we read this book, we name his birth mom. As his language ability matures, so too will our terminology. He understands tummy; he understands mummy; he kind of undestands that babies grow in tummies
the flip side of this book: there is no birth father. the characters are cc.
I feel glad that at least the birthmother has a significant and visible role in this book.