oh becky ! you were up early!
If your friends really know you and care about you, they WON'T think any less of you for going to bed early. Honestly they won't. The main bit of the holiday is what you do during the daytime anyway.
You could plan to stay up lateish one of the nights, see how it goes, then if you have a bad one, go to bed early the next night.
I know you hate them, but you could just take one zopiclone with you for "emergencies". You probably wouldn't even need it, but just knowing it was there in your sponge bag, could help you relax about it a bit.
Your friends won't think you're bonkers. When we went away with two other couples and their kids back in July, I had all bad nights but I honestly don't think they even noticed!! Remember, you need less sleep to function and be "ok" than you think. 4 hours and you will be fine.
Re. DS3 keeping you awake as in the same room - ear plugs. That's what I did in Cornwall when DD was sleeping in our room. They did work a treat.
Re. getting ill, you can't possibly control for that, so there's no point worrying about it (I know, it's hard not to!) but that could just as easily happen at home.
Try to go with a positive outlook - a change of scenery, beautiful place, chance to have a laugh with your friends.