I don't know what is 'ettiquette' really - DP has a ten year old child and when she's in school concerts (for example), DP goes along and sits with his ex to watch, then they go out for tea, stuff like that.
The thing is, my DP lives about 350 miles away from his daughter and his ex and so visiting is limited to maybe once a month, when he spends a couple of days with them (he doesn't stay in the house with them). So I can accept that it is hard for them all, but should I get so stressed that they still behave like a family?