I would talk to the airlines and see what their rules and restrictions are. If they allow a small crate, depending on the dog, get a soft-crate or an airline-approved plastic crate.
Even if you know that your dog(s) don't get sea-sick, you may still want to talk to your vet about something you can give them to settle their stomachs and keep them calm.
I also wouldn't feed them for a few hours and make sure they have plenty of opportunity to do their 'business'!
Bring some small bowls and a few bottles of water. If you notice that they are doing alright and don't get worried about the flight, you can give them small portions of food if it is a longer flight.
And in case of something happening, you may want to get them micro-chipped and have your information on their halters or collars! And if you let them lose, make sure they are secured with halters/collars they can't get out of and on leashes!