Hi There!
I have almost five year old twin boys, and we love baking together (cakes, etc.).
I've just started substituting the sugar in the recipes for dextrose powder - the kids still eat it and it tastes good to me (hubby, still addicted, says it tastes a bit bland).
Could you try asking the school if your child can substitute cane sugar for dextrose (and explain why)?
Parties - I would tend to let them eat what is there (per the "Party food is for parties" rule), as long as they understand that this is just because it's a party, iykwim?
I'm actually about to cater the twins' fifth birthday party and trying to work out if I can make the whole party fructose free without being called a wet blanket!