Baby Paint

It is easy because you don't have to choose brushes or colors. Free version with ads: https://play.google.com/store/apps/details?id=com.appyos.babypaintfree
The baby has the whole screen to paint. The tool bars of the phone are hidden. Any time the kid touch the screen the color is changed and a musical note is played. The musical instrument and the song can be changed.
To finish the drawing, parents can pull the tool bar on the screen and touch the back button. Then the drawing can be saved.
The colors can also be changed.