Hello as long as they are able to reach the trigger and shoulder the stock correctly they should be fine. My daughters started very young. Everyone has there own thoughts on how to start, but what I found helpful was a gun with a stock short enough to shoulder correctly, a gun light enough to hold, even with limited upper body strength(even if i was starting on a bench or a tripod) and for us a RED DOT sight. I stared easy, only having to line up 2 things the dot and the target, then worked backwards to a scope and iron sights. I also started with reactive targets balloons, seltzer cans shaken, and then worked down to smaller targets like cookies, crackers, and as Kraemer99 stated paintballs are great as well. Well I wish you the best with your kids and nephews, you are going to have a great time teaching and shooting with them and making some great shooting buddies along the way, my daughters are grown now and we still get together and go shooting, weather it's sporting clays, running plates or just going off roading/camping and plinking with an airgun. Enjoy these great times to come they go by very quick Eric