I have been having my son, almost 5yo, out shooting a .22 since March and he is using the peep and post sight. We started up close and a HUGE backstop, so he can see/hear the hits.
We have now moved back to about 20 yards and use a dirt hill as a back drop so he can SEE how close he is to the various targets. I like big COLORED chunks of plastic, i.e. old bowls, pieces of plastic toys, etc. If he hits one, they go flying, etc. (We shoot at a dumping ground which helps in the selection of targets!)
If my 'almost 5 yo' can get a hang of it, then an 8yo should be ok as well!
Good luck!