This app has free printable manuscript pages:
https://www.blanksheetmusic.net/
There are many useful apps available to help learn music concepts. Here are a few suggestions:
Flash Note Derby
Music for Little Mozarts
The Most Addicting Sheep Game
Note Perfect
Music Theory by musicroom.com
Music Theory Pro
Road Rhythm
Flash Notes
Music Flash Class
Practice Center (Metronome/Drum beat app)
MetroTimer (Metronome Drum beat app)
Rhythm Cat
Bastien's Notes Flash Cards
Bastien's Key Signature Flash Cards
Bastien's Interval Flash Cards
Some websites have lots of information about composers and music theory:
Many websites have fun music games:
https://www.musicindustryhowto.com/best-piano-learning-app/
http://www.aaastateofplay.com/a-musical-playground-for-kids-music-theory-lessons/
www.musiclearningcommunity.com
www.pianoeducation.org/pnokids.html
www.teoria.com
www.datadragon.com
www.quia.com/web
www.creatingmusic.com
www.easymusictheory.com
www.good-ear.com
www.happynote.com/en/music-notes.html
www.sphinxkids.org
www.musick8kids.com
trainer.thetamusic.com/en