I would agree that LW probably shouldn't have had the firepit going (or, ideally, shouldn't have had GF and her kids to whatever the gathering was). But I had a neighbor who decided that the way to get rid of excess trash (besides putting it in our bins) was to burn it,* so I've seen how seemingly "harmless" trash like paper towels can get away from a contained fire. I do believe that putting hands on anyone past the age of about three should be an absolute last resort, but in LW's shoes, I might well have done it.

*Have I mentioned how very, very happy I am not to be living there anymore?