I use youtube children's channels with my kids. They have really learned to sing using youtube. There are a lot of good channels, but here are my favorites (and the ones that my kids love):

1) Toddler World TV (http://www.youtube.com/user/toddlerworldtv).  Wonderful original music. Songs are beautifully performed in a gentle folk style. Most of the content centers around educational songs.  Highly recommended!

2) Kidstv123 (http://www.youtube.com/user/KidsTV123). Original music, nicely performed. Folk style.  Educational songs. Highly recommended.

3) Super Simple Songs (http://www.youtube.com/user/SuperSimpleSongs). Quality production. Traditional songs nicely arranged. Entertaining for the young ones...some educational stuff, too.  Recommended.