is this child endangerment/abuse

You may wish to consult with an attorney who is familiar with your Family Court for advise on the exact steps you are entitled to take.

Guns and children do not mix, it is a potentially deadly situation. A gun placed under a mattress in a house where there are children who have access to the area with the gun would most likely be considered a neglectful act (of the supervising adult), by your DSS and by the Court. You don't say what time of day the incident occured, but the fact that the adult was allegedly asleep when the children gained access to the gun is even worse. I can say that as a DSS attorney for many years, I would have recommended court action and would have asked the court, at a minimum, for an order which directed that the gun be locked up, away from the children.

What steps has the other relative taken to insure that this does not happen again? She has her own child in the house that was also put into a dangerous situation. If she has not taken steps to insure that the children cannot get the gun again, then not only would your child be in danger if returned to this house, her child is in danger as well.
While the father of your child may not have been in the relative's home at the time of the incident and was not the one who was responsible for the direct supervision of the children at the time, now he has been put on notice that there was a problem. He can no longer say I don't know that there is a problem, he has been made aware of the circumstances and should not re-expose his child to the danger. If he does re-expose his child to this situation, that could be considered neglectful.

What exactly does he mean when he says he wants to remain neutral? He is responsible for the supervision and care of his child. If he means that he intends to leave the child in the house with the gun, without taking any steps to insure the safety of his child, this is insufficent. If it means he does not wish to step into the argument that you are having with the other relative but is not going to send his child back to the home unless he is there supervising that is ok. He cannot say that it has nothing to do with him; if his child had shot someone else or been shot by the other child, he clearly would know that it does have something to do with him. He was lucky in that nothing bad happened, but as I said earlier he has been put on notice of the situation and cannot ignore it.

If there is an order which provides for the father's visitation, you cannot simply stop visitation. The Court will need to direct modifcation of the order. You can go to court and file a petition to ask the court to modify the visits or to direct the father not to take the child to this house. It would be a good idea to consult with local counsel, but if there is a continuing issue with the other house you should seek immediate assistance from your court and perhaps the DSS.