How about:
Babybel cheese - nicely "packaged" and consistent amounts (although carb-free)
Pre-cooked bacon strips - most of the big supermarkets do them, plus M&S. Yummmmmmy!
Cooked chipolata sausages (someone will have an idea for a dippy thing, like salsa, tzatziki or raita)
Small packets of nuts < my personal emergency, in my handbag snack
Small packs of sunflower seeds
Any of the low carb cookies. For example:
And have a look here:
http://www.ditchthecarbs.com/2014/05/13/low-carb-kids/ If you scroll down, there are oodles of lunchbox and snack ideas, and the recipes are usually carb-counted.
The site author also has a whole section on sweets, so things like fat bombs, another on icecream, and so on.
I subscribe to her site and get some great ideas.
I hope you all have a great trip