Put good primer on and a good base coat if you are going to have the kids "decorate" -- their artwork will last much longer. A coat of spar varnish on top will make it last even longer if you want.
I've been giving my boxes two coats of boiled linseed oil followed by a coat of alkyd primer, then whatever topcoat I have around (dark green, bluish gray, stone, and light chocolate brown at the moment, who knows what will show up this year). My nephew suggested bright yellow to complement the near John Deere green, but I'm not sure. I prefer subtle. My beekeeping friend likes to paint each box a different color, but his hives are not visible from the road like mine are.
I've had problems with primer adhesion in the past, and am hoping the linseed oil will fix that. Painting boxes takes forever, and I don't currently have heat in the shop, so have to do it either in the summer or fall, or rush in the spring.
Whatever you do, don't rush the primer and base coat -- the care you take in getting them well and properly painted will extend the lifetime of the box by a decade or so.
Peter