Children's or ???
Question: Does it fall into the children's category when it is a book they will gift to Mom when done?Answer:
A children's book is one that is written for children to read. Hence, it will have a simpler vocabulary and story that fits children's reading level and interests.
I'm not entirely certain about your question but...
If this is a book written by a child for their mother, it doesn't really have a genre. There are not enough books published like this to constitute a genre of its own.
If it's a book written for children, which the reader is supposed to to complete (e.g. fill in the blanks) and then give to their mother, then I would call it a children's book. Again, because children will be the main readership for the book. (Never mind that the mothers are supposed to get the book in the end, it's all about giving the child an experience.)
Hope that helps.