Spray Paint

There are spray paints for plastics, but another option is fingernail polish.
I used to mark things with fingernail polish on the bottom. Pink was for my daughter, blue was for my son, and black was for the toys that were put away for other kids to play with. Not that they didn't all share, but each color had it's own place for being put away and with daycare kids, the black meant it was for everyone and went in that toy chest.
Fingernail polish does not come off once it's dried. It could be scraped off with sandpaper, but it won't just peel off.
Nothing wrong with spray paint, but you could also take your son to the dollar store even and let him pick out some colors and make any kind of design he wants, from flowers to squiggly lines. They even have glitter nail polish. I bet he could choose lots of colors for less than a can of spray paint and have fun making it more personal. Even writing her name on it.

Just a suggestion.

Have fun!

Just use, Sharpie markers on it, to color it/put designs on it/her name.
that is what we do,
Less hassle than paint.
Paint on plastic will peel off.

I would get some really cool stickers (the kind that REALLY stick) and let your son decorate it. I wouldn't paint it - seems like WAY more hassle than it's worth :)

Good luck!

There is spray paint available specifically for plastics.

Krylon has a spray paint for plastics: http://www.krylon.com/products/categories/plastic/ I think you can get it at Lowes or Home Depot.

Good luck!

Get some girl stickers for it.

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I wouldn't use spray paint because in time it will chip. Use an oil based paint as that will stick to anything and last for a long long time. You can use artists oil paints but know that they can take up to 2 weeks to dry, depending on the humiditiy where you are. Or use oil based paints from the hardware store. You can get sample size cans for about 3 bucks.

I have a degree in painting, trust me that oil is a good bet. :)