I agree with you, and while I think 8 euros for a bottle of Coke is outrageous (worse than Paris prices!), I'm actually cutting the OP a bit of slack in this case. The limited amount of time they had probably would not have been enough to seek out a local place, and the risk of cranky children spoiling things for everyone was probably a good enough reason to spend whatever was needed and chalk it up as an experience (or a learning experience).

Or maybe local "fast" foods were indeed available and they just opted for familiarity instead, who knows?