I'd talk to my parents well beforehand and ask them if they'd consider one of them swapping seats if DH 'acted up'.
Imagine the humiliation of OP actually having to broach this with her parents! What would they think? Their daughter asks them to be on hand to help her with the children, when and if (but most likely when) their son-in-law and father to their grandchildren decides to refuse to parent his own children on the flight.
I know what they'd think. They'd think he was a shit and an awful partner. I thnk the likelihood that OP puts this to her parents as a request is vanishingly small.
Most of these suggestions are practical, pragmatic suggestions - kindly meant - for coping around what is absolutely fucking outrageous behaviour from OP's DH.
He said if I'm going he will come too. I just know that he will be stressed and miserable on the flight, which will make me stressed and miserable, and will moan any time DC get grumpy. I honestly think it will be easier and less stressful if he's not there! Am I being a massive b!tch? I know he's trying to be supportive
He's not trying to be supportive. He's giving himself a way to renege on any promises to behave with decency. He's already used the 'threat' of not coming at all - oh dear that didn't work as OP said 'Ok that's fine I'll take the DC on my own'. Great. So then he changes tack. 'OK I'll go with you. It will be shit and we'll be stressed but I had better go to help you cope'. Er, no. OP is now faced with him coming along and making her feel massively more stressed! And she now manages to feel like a bitch as well! Even more of a result for him. Now, he's going to come and reserves the right to make her life a misery. And her arm is right up her back, as the only opportunity to get extra help is from her parents - but if she asks them for help, it will be glaringly obvious to them that she is married to a shit, and that is incredibly humiliating for her.
I think we are looking at this from the wrong end of the telescope. It's not about 'how to cope' with DH on a long haul flight and subsequent family holiday, it's 'how the fuck does anyone think he is reasonable?' and go from there.