Well, essentially what you want to do is 'tole' painting (a centuries old form of folk art that is generally said to have originated in France, although I have also read it has been traced further back to China, where furniture was painted with decorative art). History lesson aside, it's a great project for the kids!

I'd encourage you to check the Tole Painting board here as well. You'll probably find someone who's done a mailbox. I've done lots of tole and have worked on some metals, but no mailboxes. The 'general' process is: seal the piece, apply the decorative paint, then seal with a finish sealer. If it were me, and the surface is already glossy, I'd probably skip the first (prep) coat of sealer. (It's done to seal porous surfaces, such as bare wood or fabric, so the decorative paint will 'glide' onto the surface rather than absorb into it. Since you're working on a mailbox it's not porous and won't suck up the decorative paint like bare wood or fabric.) However, if the mailbox is 'chalky', then I'd do the prep sealer. You can get a tole sealer at any craft store, just be sure to get one that says it won't yellow with age. I prefer Blair sealer products for that reason, sometimes hard to find, but there are other good ones. I have a birdhouse I painted morning glories on, the paint still looks good and a little wren comes back to it every year. But it hangs in a tree only in warm months, so is not exposed to beating sun, ice or road salt splashed, etc. So I'm not sure I'd use 'traditional' craft paint (Delta, Deco Art). I would suggest a bit sturdier paint made for exterior projects (Deco Art Patio Paints) or the small cans of rustoleum (lots of color choices and it's made for metals/exterior use). For finer detail areas, you can even use auto touch up paint. That stuff tends to be thin and set up quickly, so it's not as 'workable' as prior products mentioned. (A practice run might not be a bad idea, so the kids can get a 'feel' for the paint they are working with so they'll have better success working on the mailbox. Shiny poster board would work.) The most critical part is the final sealer. Wouldn't use polyurethane (more suited to interior). Would suggest a heavy duty sealer (such as marine varnish) that can grab to metal, etc. and still withstand the elements. Another tip you might want to consider: paste wax. I bought some expensive wrought iron pieces for outdoors and applying a good coat of paste wax to help repel water, preserve and protect it was suggested. But that would be the final step after sealing (and allowing sealer to dry completely).

I enjoyed tole painting for years. The key to keeping the exterior finishes from being harmed by the weather is a coat or two of polyurethane after you are finished with your design. I used waterbased as the oilbase leaves a yellow cast.
Any acrylic paint such as Ceramcoat etc. will work for your design on the mailbox as it is, no sanding involved.
Hope this helps....Enjoy!

Thank you very much for your suggestions!
I had not thought of a sealer, that may be the answer after all for preventing the paint from peeling off.
Great!!

The mailboxes are for my daughters' gardens, they are going to decorate them and then store their gardening tools in them.

Thanks again!

The mailboxes are for my daughters' gardens, they are going to decorate them and then store their gardening tools in them. Now that's a great idea! (If you only knew how many spades etc. I've lost only to find them laying under a plant or mulch a year or two later, all rusty ;)