I agree with above comment to wait on painting until they get some stick time in the air with it. I didn't add color to the first 2 planes i built years ago. Good thing too because they didn't last. lol What kits did you get? Pro Tip - Before you build, take the parts and trace them out on some poster board. That way when your kids crash, you have templates to make repair parts or build a whole new plane.
But, if/when you do add color, there's many different ways. As stated above, colored tape is quick and easy, but not my personal favorite.
When painting the FT kits, make sure you scuff the paper with some fine grit sandpaper or some steel wool. The FT kits have a waxy coating on them that makes them water resistant, this coating prevents the paint from sticking. Scuffing the surface gives the paint something to stick too. I use Rustoleum 2x spray paint. Make sure you use multiple light coats and NOT try to get all covered with one coat. If you soak it with paint it will delaminate the paper from the foam.
Blue painters tape works good for crisp lines but make sure you remove some of the stickiness by putting the tape on your shirt a couple times before you apply to the parts. This is especially important when applying the painters tape on already painted parts.
I also like to print off decals. If you have a standard inkjet printer, just use google image to find what you want. I personally use Microsoft paint to get the image the exact size I want as I don't really have any other software. Then you print it. I then take the printed picture and using some thin packing tape, cover the decal front and back, and then cut it out and apply with some spray glue. This is how I did the decals on the attached pictures. For bigger decals like the decals on my 3D XL wings, I used the print in tile function on Microsoft paint and just cut/taped them together. The result is actually pretty good for how easy it is.
