From some of my experence and what i have seen on this site here is my 2 cents.
1- Try and see what they like, either by pictures, books, tv, online, hobby shop or hobby shows.
2- Let them pick what they want, if they want to do a f-16 first then a battleship, let them have a go at it. With colors, they are always right.
3- Start off easy, if it becomes a chore to do a model then guess what? they'll no longer want to build models. The snap togeather ones i think would be a good start but i have seen warnings about some being harder then the ususal level 2 skill. And if they lose intrest then let them start a new one, i find that i lose intrest in models i'm halfway through and it's not longer fun to work on them so i move on.
4- Rember, this is ment to be fun. Don't try and push it on them. It could be that your kid simply does not like to build models but would rather sports.
Mongol General: Conan, What is best in life?
Conan: To crush your enemies, see them driven befor you, and hear the lamentations of the woman!