I presume that these nephews are minors.
If so, one of their parents would have too do the actual gun buying. The easiest way would be for the boys, along with dad or mom (or both) to go to a gunshop and pick out what to buy. They could then send you the information concerning what the costs would be, including taxes and whatever. You could then send the parents a check or money order to cover the specified amount.
Another way, which would likely be much more expensive, would be for you to buy the guns, and then have the dealer ship them to another FFL in Chattanooga. One of the boys' parents would then complete the required paperwork and take delivery of the guns from that dealer. Expect to pay fees to both dealers, and shipping costs as well.
As for ammunitiion, send the parents a check once and awhile, and they can purchase the ammunition.