How to book a return flight for a child when she is flying outbound with one parent and inbound with the other?

Mum flies from Canada, where she lives, to Belgium with child. Mum then returns to Canada on her own after a couple of weeks. Dad flies from Belgium, where he lives, to Canada with the daughter after a few more weeks. How can I (the dad) book this on-line without paying for 2 return tickets for the child, I can't seem to do it on sites such as Opodo, Expedia, etc? Thanks in advanceHow to book a return flight for a child when she is flying outbound with one parent and inbound with the other?
Find the flight you want on Expedia or kayak and book it directly with the airline it shouldnt be a problem