It sounds like you need to work out how to deal with the food situation on holiday, focus on trying to have a good time with your DDs .. It sounds like you need to de srtress.. Holidays are good because you are usually relaxing and doing plenty of walking etc.
I wouldn't panic about DDs eating a doughnut or two on holiday. If you are setting the tone at home by eating healthily and getting enough exercise and making food that is good for them... they will see the efforts you are making.
It sounds like you already have a good idea of what you can and can't eat at the moment... so maybe do some searches to find places where you can eat the stuff you are allowed to have... or a trip to the supermarket to buy things that will help. There maybe some foods you can have as a bit of a treat, so plan in some in the right quantities..
But if not, then reflect on how you said the plan you were following was working, which is brilliant. Its also good that you've had some medical diagnosis, even if further tests are needed because you can get cracking on putting things right.
You have all of August before you start your new course in September, which will lead to new opportunities.. Try to get a good nights sleep, relax as much as you can or explore where you are with your family.
Don't worry too much about what the rest of them are eating on holiday.. Just make some plans for how you can make your dietary needs fit in and improve everyone's health inthe long term. There's probably associations which deal with these conditions, which have dietary advice, or support.
You could also ask your GP to refer you to a dietician. They often have more time to listen and advise on specifics.