Happy Easter Debbie
Im not sure what regime your 12 year old is on but if on basal bolus I would prob give them the choc at the end of a meal and calculate it into the insulin dose. Or, if you arent going to be eating any time soon and they must have it (and it is Easter, and they are 12!), I would just give them what they want and give the insulin for it.
If they are on premixed insulin then its slightly less flexible. In that case I would just give them a bit of what they want, maybe close to the next dose or just go with the higher bg for a while in order to get a treat. You can bring it back down by next meal time
85% choc is less carby/sugary than milk chocolate, which is a shame for me cause I hate the stuff and would swap it for a dairy milk any day of the week!
One day wont tip the balance of control, so maybe just for one day......