I know this is too late for Christmas...
If the guns are stored responsibly, that is, locked and totally inaccessible, I don't see what the problem is.
I totally believe that children who aren't raised around guns should not be trusted around them. But keeping them locked in a gun safe is not the same as trusting kids around them.
Our guns are stored unloaded in a gun safe, which is installed in a hidden location. The key is always on DH's person, not somewhere a kid could get a hold of it. The ammo is locked and in a separate hidden location - I don't even know where it is. Let's say an untrustworthy kid were visiting our house. The kids would have to be left totally unsupervised for hours to even locate both the gun safe and the ammo, and even then the only way for them to be that unsupervised would be if no adults were home, in which case the keys to the safes would not even be on the premises.
I agree that OP's DH is looking for an excuse not to visit. Either that, or he has an unrealistic phobia of guns. Guns can be dangerous in the wrong hands, yes, but so can knives, stoves, hot water heaters, fire places, and bath tubs. A lot more young children die from drowning every year than from accidental gun shot wounds; would your husband feel the same way if your mother owned a swimming pool, fish pond, jacuzzi, bath tub, or cleaning bucket? Or lived near a body of water? Proper storage of the guns combined with proper supervision will eliminate the risk of a gun related injury. All cases of accidental child shootings (meaning, a child gets a hold of a gun and shoots himself or another child) involve either improper storage or improper supervision; usually both.
If you call your mother and she tells you she's unwilling to lock up her guns for the visit, or if she refuses to not carry one on her person and you and/or your DH are uncomfortable with that, then that's a different story entirely. In that situation you are justified in not visiting.