Before you think of their permanent visa or send your kids for exit (as their visa is near expiry and you are short of time), you can try to apply for another month extension. I had a six month visa before for my son. Unfortunately, i was not able to get a residence permit for him during that 6 months period. But I was allowed for another month extension. Try your luck to extend their visit visa, then at the same time proceed processing with their residence visa.