Insurers didn't say they wouldn't take ANY children; they simply said they wouldn't write policies for ONLY a child in a family; they want at least one parent w/that child; or, presumably, all the children. In group plans; by definition; a child must be included with at least one parent, as well as his/her brothers and sisters.
What the insurance companies are, ostensibly, guarding against is severe selection against them. Say a family or Medicaid has a single (really) sick child; it is VERY likely a provider would pay for that child's coverage on a private plan, because that provider would stand to make much more than he could under Medicaid - hypothetical, but I can imagine that is the kind of abuse the companies are worried about; the family has to have more skin in the game, essentially.
Also, under group policies; the premium difference is exponential vs. what a single child on a single policy would produce (children are, by far, the least expensive people to insure - although inexpensive treatments are common (vaccinations, ear infections, etc...) REALLY expensive stuff is very rare - they haven't had time to screw up their bodies like adults have.
I might or might not have answered your actual question. Let me know.