Some recommendations of mine are the TC Omega, TC Black Diamond, NEF Sidekick, Lyman Mustang, Knight Revolution, and Knight Wolverine. I feel that for the money, these are the best you can buy. All mentioned will serve you well, be very accurate, and last a lifetime. They are simple to load and clean. I own a TC Wods Rifle(cheap Black Diamond) and a TC Omega. I absolutely love both guns and don't think it gets much better than the Omega. Cabelas has them for $300 right now, which is very reasonable. I got mine for $260 in blue/black with discounts. Look at local Wal Marts. You might get lucky and find a cheap Omega or something.

You should look into those guns and do some resarch. All are top of the line and will please you. Buy whichever gun you feel is best. Good luck getting your nephews into the sport. I am 15 myself and have been MLing for 3 years approx. You'll get hooked--guaranteed.

By the way, some people do have great luck with CVA Kodiak MLers. You should look at it as an option. Good luck deciding.