I am affraid it is not a percentage discount but an amount discount, and the discount amount is not the same for adult and child tickets.
Could you help us develop the custom script?
please try and purchase a ticket for 9 April 2017 ( https://tickets.universe.dk/billetkalender/ ) and apply the coupon code ‘sonderjyske’ at checkout. (PS. the coupon is not valid for that day!)
This will showcase the issue with the quick clearing of coupon notification messages.