They were 9 and eleven in 2003. They could have been trained, but it seems unlikely that they would have laid low this entire time, particularly since they could have had a much bigger impact as child bombers in school at that time, which Chechnya was still just a few fights away from breaking free.
But if this is true, then they've been groomed this entire time. I expect, however, it would have been hard to train them, and then hold them back as teenagers. More likely that things weren't going well for them in the US very recently, and they sought out purpose in lives they didn't feel fit them, and found it online with radicals.
Now, see, they should have become Mormons instead..