I have created a new topic in the Resources folder with the .kra files for size 8.5 x 8.5 inches which you should be able to download from Google Drive. The thread is here.
It’s quite a large programme, but once you get the hang of it, it’s simple enough. I would say use the layers to your advantage; have one for the line art, one underneath for the colours, different layers for different objects so you can move them around without having to re-draw the whole thing. You’ll get the hang of it when you have a play about.
Oh, and perhaps, download some external fonts to get some better suited for children’s books. I like this website for free fonts. Be sure to chose from the green ticket ones for free commercial use.
Edit: When you create a new file, ALWAYS ‘save as’ as the .kra file. If you save it as a jpg or png, then when you open it again you’ll most likely find it’s gone all flat and your layers have merged. Always use the .kra files for editing and when you’re finished and ready to save the completed picture, go to File - Export and export as JPG or PNG etc. Always leave your original .KRA file as you may want to go back and change something.