Hi - I was a Primary TA / helper for over twenty years, and did a lot of ICT, including club activities with children, and training for our other 19 TAs (I worked in computers before getting into education).
There are two main types of graphic program on computers: "bit maps" where each pixel is coloured, and 'saved' which takes a lot of space, and this is what Paint is.
The other kind of program is a "vector drawing program", where the image is made from a sequence of 'shapes' and when it is 'saved' it is the INSTRUCTIONS to recreate the image that are saved, taking up much less space, and also can be modified more easily.
"Clip art" images can be of either type. A "drawing" program can be enlarged with no loss of quality, whereas a "bit map" will show up the individual pixels as it is enlarged.
[I apologise if you know all this already.]
So, ideally you want a vector drawing type of image, and may be able to flood-fill areas. If you are stuck with a bit map image, you should still be able to 'paint over' it, but will need to use an appropriate size 'brush', or possibly 'spray' tool. (If you zoom in very close to a spray-tool you should see how the pixels are coloured). There may also be a 'wash' facility, where you can see colours underneath through the wash (like a pale water colour).
I have never liked any MS programs, and seldom use them. But I am retired now, and don't know what the up-to-date programs in school currently are.
If you tell me the Yr group involved, and whether my 'lecture' has made any sense to you, I will come back later to see if I can provide more concrete advice.