When I was at primary school everyone had to learn the recorder, but I definitely don't think it is the easiest instrument to learn. Not only do you need good breathing control (as with all instruments that require blowing into), but you can't see the holes you have cover.
I think the keyboard is a good choice for a beginner. Unlike a piano, you don't need to worry about the pedals, and if it is an electric keyboard, you can learn the sounds other instruments make too, and even receive 'electronic lessons' on how to play.
African drums can also be easier to learn than drums for rock music, and can produce nice sounds without much knowledge of what you are doing.
Someone in a parenting group I'm in suggested the ukelele as a good instrument for kids to learn. Apparently it's pretty easy and you can get a decent one that stays in tune pretty cheaply. It's got to be better than the recorder!
I currently play the piano, Acoustic guitar, bass guitar, recorder and drum set. I've been playing the piano the longest. The piano is pretty simple but I think percussion instruments especially the drum is the easiest and most fun for kids to learn.