I'm not familiar with that book so this is not a direct comment on the storyline, but children are far more vulnerable to radiation than adults are. The younger they are, the more sensitive they are and the more likely they are to suffer short and long term effects.

That's why my main advice regarding Ironwood's question was to consider just getting the heck away, for the sake of his kids. Evacuate, and if you need to come back to the house later on, then just have the adults return to the hot zone. Or better yet (in a cold, calculating, amoral way), send the grandparents back to retrieve things.