Go! Animate is a mostly free animation program. You (and the kids) do have to be careful NOT to choose any of the choices that cost money, but given that it's rather cool! You can upload your own voice over and background music, and there are many 'community' backgrounds, images, sound, etc. It's fairly easy to learn and soon you too will be creating your own animated cartoons.
GoAnimate.com: Virtual Classroom by PJMSLAB
Like it? Create your own at GoAnimate.com. It's free and fun!