@samuvk...The trouble with loading a PDF of your kids ebook into Kindle Kids Book Creator(KKBC) is that each page(text and images) in your PDF file will automatically be transformed into a full-page jpeg image. That's a BIG hit on your ebook file size.
You might want to investigate creating your kid's ebook as a reflowable ebook using
Kindle Create(free app download from Amazon). If you go this route, you just load your
properly prepared Word doc into
Kindle Create(choose "Reflowable"). I would also advise you to insert/resize all your images in Kindle Create and not Word. You can also create full-bleed width images in KC by clicking on an image and selecting
Image Properties panel > Size > Full and then choosing
Bleed for ebook > Place to edge. For max screen width your images should be at least 1200 pixels wide and full page images should be at least 1200 x 1800 pixels in size(with an aspect ratio of 1:1.5). Also bear in mind that you will not be able to create pop-up text boxes in Kindle Create(like you can in KKBC).
And since it's really your images that are causing your file size problem, you might want to consider using the
TinyPNG website to reduce the file size of all your images
before your insert them into your reflowable ebook using KC. TinyPNG uses lossless compression to reduce the file size of your images which means that, after compression, image quality will always be preserved.
Using TinyPNG to compress your images and creating a reflowable ebook using KC as described will probably more than halve your ebook's current file size.