I don't have experience in this area, but the soprano uke seems well suited for a child (given the size).

If you were in a bind you could always get a book of children's songs with guitar chords and just use the chord names along with a ukulele chordbook.

Hey Twinkletoes, I just posted a link for a book that I think is great for beginners and kids. Check it out:
link

I'll second the alligator boogaloo website (which also publishes the book that ukemike recommends).

definatly start your child with the soprano.