When they are young (pre-school) start with the "friendly" classic rock that has a lot of harmony: Beach Boys, Crosby Stills Nash, the Beatles.
Then slowly introduce things they find interesting. The beatles are great bc you can bridge to "harder" guitar driven rock but its still friendly bc is the beatles.
And then see where it takes them. If they prefer strong vocals, well, that's easy and you can bring them from Elton to Billy Joel all the way up through modern vocalists.
If they like the pounding drums and guitars, I found that that Led Zep and Nirvana are actually pretty accessible to elementary aged kids
And then, if they really love it, get them some used equipment off marketplace and let them make as much noise as they want in the basement.
It's loud, but its totally worth it
