Before the summer holidays my son won a free ticke...

Before the summer holidays my son won a free ticket at school on the condition that an adult also bought a ticket.
On Sunday 13/08 we went to Bobbejaanland, my wife had ordered tickets online for herself for me and our daughter.
Once there my wife had to go to the counters with the free ticket, there she had to pay 16 euros !! ??
Are those then free tickets? That is just pure deception. Many people will be bothered by this my mail to Test Purchase has already been sent.