I think I should not have used the phrase "can't side with LW here" because I seem to have given the impression that I'm excusing the mother's carelessness and horrible guest behavior. I'm not at all. I actually think LW should have kicked them out when he saw her allowing her kid to throw things into the fire, before he felt it necessary to grab the kid. However

GF should have been parenting her kids full stop, not drinking beer with others

she absolutely should have been but by then LW knew she wouldn't, which kind of allows her to hold LW and his firepit hostage using her and her children's bad behavior. It's not fair that he can't use his firepit around her, but I think it was not a good idea to use his firepit around her knowing how terribly she and her children behave.