It has always been my opinion that kids should have the colour they want in their room, but within a reasonable intensity! Not the screaming yellow or lime green they are currently waving at you, but a toned down version of that. I always tell my clients, 'your home is your decorated, personal space but your child's bedroom is their personal space, so it should be an expression of their personality'.
I'm showing this first room (above) simply because I love it, but most people want colourfull walls for their children's rooms.
Sometimes a client is attached to having their child's room flow with all the colours in the house. Since most people don't paint their house varying shades of primary colours then your child's room needs to look like a sophisticated room to grow up in. Then it could be like this one (above) by David Kleinberg.
2005-60 Pink Pearl (above) is my favourite pink for girly girls, it's soft and far from being a 'bubble-gum pink' which you get if you don't gray it down enough to begin with (room by Phoebe Howard)! And here's another room without wall colour (below) it's just a nicely decorated space!
There is a big urban legend in the colour world that yellow makes babies cry. This is completely false. Lee Eisman said that a colour expert once announced this in a talk and the press picked it up and went with it. She said he completely made it up and said it simply for drama.
My opinion on why this myth is out there, is that people have so much trouble with yellow (see my previous post about that) and/or if you painted any bedroom a bright screaming, primary yellow, well we would all cry in a room THAT yellow. If you want just a nice soft, butter yellow like this one, try Straw from Benjamin Moore, 2154-50. It will give you this effect (below):
2071-60 Lily Lavender is the perfect lavender for a little girl that loves purple! The addition of the fuscia drawers in this room really adds drama and colour!
My favourite kids room green? Dill Pickle, 2147-40. It's a great yellow/green which I've specified so much the paint store's know it's my colour chart before they look at the name.