The actual amount of sugar from 'contamination' must be minimal - think about it, it all comes down the same tube so if it was significant, every drink you ordered would taste like Coke.
What is far more likely is either the person pouring the drinks isn't paying attention and pressing the wrong button, or they've got the wrong syrup hooked up the diet button. That's assuming, of course, it's the coke that's the problem and not the change to your daughter's routine from half term.
It's very easy to check if your drink is diet or not. Spill a little on your fingers and let it dry. If it's very sticky, it's regular. If it's not, it's diet.
I wouldn't avoid draught Coke on the basis it could be contaminated because the contamination would be minimal at best, we're talking like 0.1g here. Syrup mix-ups are more of a problem so I always watch carefully when they're being poured - typically diet coke is a lower button than regular. Also, when you've been drinking diet Coke as long as I have, you can spot the regular stuff instantly because it makes your teeth go furry.