We all forget song lyrics, especially if we haven't sung the song in a while, or in years for that matter.
The website below includes many songs that are used in the regular classroom and in the music classroom. This particular page features folk songs. We sing many of the songs listed on this link:
This website is very helpful if you or your child find yourselves singing at home and forgetting the lyrics to particular songs. There are also short musical excerpts to listen to if you or your child need a reminder of the melody (melody: a series of pitches that move up, down and stay the same; the main part of the song).
Have fun singing!