Utah's child welfare agency (and what an oxymoron that term is in most cases) tried this a few years ago and found they were in violation of State Preemption. They had no choice but to relent. They now go no farther than to require that guns be kept locked up if not actually being carried.
While one might suppose that rules against having guns in the home are based on sincere safety concerns, I have to wonder how much they are also (or instead) based on some rather ugly--if un-admitted--bigotries against gun owners. Does the Oklahoma child welfare agency ban alcohol inside foster homes? Does it ban those who might be on some kind of pain medication from being foster parents? What about homes with swimming pools? Or motorcycles? Do they actually require that kitchen knives or household chemicals be kept locked up?
Lots of household items pose a much higher risk to the safety of adolescents and children than do firearms.
I suspect that in most cases, these types of rules about guns have far more to do with "those kind of people" being entrusted with children, than with any real safety issues.