My twins are 7 and have been shooting for a little more than 2 years now. Dad and I started them off with .22's like you mentioned to give them the basics with aiming, firearms handling, and trigger control. It wasn't long at all, a couple of weeks actually, that they were shooting my sisters .243. Now they shoot their own .7mm.08's that the old man built for them. They are a little big for their age. They actually look like a couple of 9 year old linebackers. If you own a .243, I would get them on that before a muzzleloader. If not, and a muzzleloader is all you have, then I would really suggest you load no less than 40 grains for safety. If it is an inline, then I am not really qualified as I know little about them. But I have been shooting sidelocks for quite a few years now and 25 grains, even T7 3f, would be, in my opinion anyway, a bit too lite to safely depend on.