QUESTION for EXPERTS !

Dumb me : I too quickly booked a GVA-BDS for me and 1 of my child. Now my second child is free to join us.
But the system dosn't allow me to add a person to an existing ticket. And when i try to make a new booking, the system (obviousely) dosn't allow you to book only 1 child.
Do I book an adult ticket for him (he is 4) and call it a day ? Would there be any issue at the airport?
Or do I try to call easyjet to try to fix thing (adding a child to my existing ticket / or changing my eventual new booking from adult to child) ?

Thanks to you all experts !