How long till you go? Have you got time to order one of those scavenger hunt type books for the place you're going? Or an airport related one? Those really help - even my 2 year old can get distracted by looking for a particular thing.
Take fruit or yoghurt pouches for take-off and landing.
And bribe them with all the stickers / food / screen time they want. I always take a bunch of cheap little toys - crucially, these are new. They get a couple each when out for dinner on hols, and it's something to play with. Also, so many sticker books!
You might also give them a little bit of money each to spend on whatever they want whilst away. Sure, they'll need help. And sure, you'll come back with a bunch of nonsense, but they'll enjoy it,
We also tag team at the beach or pool, so we get an hour off to read / snooze / whatever. We're both still immediately there if the shit hits the fan, but if it's your hour off all the random questions and requests are the other parent's to deal with.
Finally, tell DH they don't need their shoes in the back garden, as long as that's safe. Some battles just aren't worth it!
You'll be grand, they'll nap, and you'll have a good time, at least for part of the time,