N.S.
You don't say "how" you found out there are guns in the house. How you learned this information can play a big role in your deciding whether you want to associate with these people or not.
Did the husband mention he hunts? Was it your daughter who may have seen them laying around while playing? Was it mom mentioning they have a gun for "protection" in general conversation?
For myself, if I found out there were guns in the house from my daughter after she had a playdate over there, without question, I'd be pulling the plug permanently on future playdates. The fact that there was no disclosure, and careless behavior being the reason.
If you found out directly from the parents they bought one for "protection"..I'd say no way, no play. I explain later.
And lastly, I might be more flexible if they're a hunting family, it would depend bigtime on how they store their guns and how well I know the family and their overall temperament and value system.
It sounds like you don't know them very well. That would be a big red flag for me. There's too many questions about them. Certainly if they don't use gunlocks or store the gun in a safe, you're asking for trouble. But I'd also consider whether there is strife in the home, whether they have volatile personalities, whether or not they have failed to respect your wishes regarding your daughter in the past. If they're lacking in any of these areas, you know there's a greater chance that these guns might be used by either the adults or the children in a very unsafe way.
My big beef with the guns for "protection" is that most people who keep them for this reason are not going to store the gun and ammunition separately, let alone use gun locks because they're going to want quick access in the case of an emergency. Shoe boxes versus safes are the popular storage place, and statisics show that most gun owners who have them for "protection" either wind up having the weapon used against them, or having an accident in the home.
If they are serious hunters, most sportsmen/women have a whole different outlook on guns. Most hunting families I know have gun cabinets, and a whole culture of safety and respect is built around their use. Most hunting families put their children through safety classes, and the guns are mostly used in supervised situations. I'm not saying accidents either in the home or field due to carelessness, dangerous behaviors such as drinking, or domestic issues don't happen among hunting families, but I think it is less likely, because as I mentioned earlier, it seems among hunting families guns seem to be viewed and handled differently.
I maybe out on a limb here, but if this family is a true "hunting family" they'd probably be more respectful of your concerns and wishes than not. All you can do is try to talk to them. But I still stress, if they don't already have safety measures in place, I'd be questioning whether I'd want to associate with them at all since this may indicate their lack of concern for safety of not only their children, but anyone at all.
If they're not responsible now, they will never be responsible, nor will they ever be suitable for friendship for your daughter. How could you ever trust her safety in any other circumstance, if they can't handle something that's a no brainer if you're going to have guns in the house.
Realistically, you will not be able to keep your daughter from going to their house if you continue a relationship with them. Sooner or later they're going to invite, or eventually become insulted when you continually turn their invites down. You'll have to decide now, what you're willing to tolerate if anything at all.
Bottomline, if they are leaving guns in plain view around the house, whether they're for sport or safety, chances are they're not going to change how they handle or store them because you ask them to. Especially if they're of a temperament where they don't like to be questioned or challenged. Guns is an explosive (no pun intended) topic both politically and philosophically. If our legislators can't agree, I can only say be ready for bristled neck hairs and clenched teeth if you bring it up to them.
Good luck!