I think that this is a brilliant series. I also think that this discussion underlines that we need a hell of a lot more books for children featuring women in STEM, as any book is expected to do everything and represent everybody, which isn't the case with other books.
I also think what almondpudding and others have said - that it's important to have mothers depicted as actual people - is crucial. Women can be both mothers and in STEM, not just random people who happen to be female but because there's only one of them in the workplace everyone else pretends they are "just one of the boys". I was slightly shocked and utterly delighted, for example, on seeing the Royal Astronomical Society portrait series of women fellows that they commissioned to mark the centenary of women being admitted to the society, one of the portraits was a young astrophysicist who was very, very pregnant when her portrait was taken. And there she is, for all eternity, pictured being a working astrophysicist while in a very obviously, indubitably female body, and the sky didn't fall in! It was so great to see.
And I saw a science show a few months ago where I only realised one of the women speakers was pregnant when at the beginning of her presentation (on engineering) she said "if I pause when speaking, it's because this baby is kicking really hard". And that was great, too! It wasn't a HERE I AM BEING A PREGNANT WOMAN ON STAGE, it was just a throwaway comment by one of a series of brilliant women speakers, to explain why she might pause from time to time. Women's lives as normal and not to be hidden away in case the women who have those lives aren't taken seriously.
I went off to order "My Mummy Is An Engineer" in the middle of writing this post (for my baby nephew) and it's got excellent reviews - although, rather fabulously, one reviewer on amazon who loved it nevertheless only gave it three stars because it didn't mention chemical engineering!