Here's what I would do. He's going, so firstly just accept it and put one in the bank for yourself at a later date.
Then, sell the idea of a special mummy bedtime to the kids. This will include the easiest possible tea and bedtime routine. Maybe picnic tea with a new DVD and chocolate buttons for the most nicely behaved children. Bugger the bath if that's tricky. DH can take them swimming the next day. Nice easy ready meal and a glass of wine for you. If you're a lovely smiley mummy they're a lot likely to be lovely smiley children.
Finish with their absolute favourite bedtime stories. If necessary let them fall asleep in pjs in front of the tv.
If they have dietary restrictions etc you can substitute your own ideas.
This kind of approach always worked well for me ie taking toddlers on the train for several hours.
You really do need to be able to do this on your own, I'm sure you can. So does he so get planning
.
Changing your attitude to the whole thing will honestly make it 100 times easier.
As an aside have you considered claiming DLA for any of the dc if their needs are so great? Hope it goes well for all of you.
Ps I don't think it would be unreasonable to ask him to be back by a certain time depending on what time he goes and how far it is.