Did he fill out DS-230 for each child and submit it to KCC?
Did the kids go to the consulate and receive immigrant visas with the father?
if yes, they need to enter the US with the immigrant visas in order to get their GC.
If no, how can the father possibly expect that the kids will get GCs without getting immigrant visas first?
The other way is to file a I-130 petition and wait for 6 years for them to immigrate.
THere is also a following-to-join, but it might take too long... and the kids may not get their visas before Sep 30.