I wouldn't let my kids have lemonade either. Why do people think lemonade is so much better than other soft drinks?
My son is 5 and has had one sip of soft drink when he was 2 and his Aunty gave it to him not knowing he shouldn't be having it. My 2 year old has never had it. I don't see the need to give it to them and I don't want to make it a part of their lifestyle. If they want a treat in the form of a drink I would opt for juice or a milkshake.
I can't believe that it is so outrageous for the OP to think soft drink as a reward at school is a bad thing.