Assuming the children are unmarried, whether they have aged out as derivative beneficiaries or not is determined by the formula in the Child Status Protection Act (CSPA). Specifically, the amount of time the petition was pending (time from when the I-130 petition was filed until it was approved) is subtracted from their age when determining whether they are under 21. To take advantage of the CSPA rules, they must "seek to acquire" the immigrant visa within 1 year of it becoming available.
So if you filed 13 years ago, that was around 2004, and you said it was approved in 2009, so it was pending for about 5 years, in which case they haven't aged out as long as they're under 26 (this is approximate because we don't have exact dates; you should do the calculation with the exact dates). If they were 10 and 14 13 years ago, then they would be about 23 and 27 now, which means the younger one probably hasn't aged out, but the older one might have (again, you will need to do the calculation with the exact dates).
I am not sure exactly how to contact them to add the younger child (and the older child if the calculation shows he hasn't aged out) to the case, but you can try writing a letter to NVC requesting a CSPA evaluation for the younger child, and to request an invoice to pay the fee for him.