Making pictures like those shown in Drew's links isn't trivial. You and your kids will need a capable tool to do so.
Many Linux distros have The Gimp pre-configured. There are also [link|http://gimp-win.sourceforge.net/stable.html|Windows installers] for it and its dependencies. (Note the Help file is separate.)
Feel free to buy Photoshop. But don't give up after one attempt on something free.
Good luck.
Cheers,
Scott.
(Who has [link|http://z.iwethey.org/forums/render/content/show?contentid=188271|very little experience] with The Gimp.)