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Assuming these rates include an airfare component, the airfare is likely the cause. Child airfares tend to be a discounted full fare, so say if full economy (Y) is $1000, a child's 25% fare would be $250. However the airline could offer a highly discounted XYZ fare of $200. But a child traveling with two parents who booked XYZ fares would still pay the same child's rate. (these fares are all ficticious simply to illustrate the concept)
Could you book the child as an adult, probably ok for the airline, but could effect the hotel component as you would have three "adults" in your room.