Why do children sometimes cost more than adults when booking vacations?

Question

We are checking out some all-inclusive vacation packages, and we are noticing that in some cases our 2 year old costs more than one of us (the parents). We've found several hotels where this is the case. Interestingly, at least one of them charged more for the kid than an adult for a particular departure date, but when choosing to depart a day earlier or later the price for the child would be less than that of an adult.

Here's an example:

Price Summary

2 Adult(s) x $719.00 = $1438.00
1 Child (2 yrs) x $799.00 = $799.00
2 x Adult taxes & fees x $389.39 = $778.78
1 x Child (2 yrs) taxes & fees x $389.39 = $389.39
Total Insurance Cost $0.00
Total Price $3405.17

This is going to Grand Bahia Principe Coba in Mexico from Calgary, leaving on January 6th, 2016 for 7 days. This particular copy-paste is from RedTag.ca, but we checked one or two hotels on other websites and they was no difference. What could be the reason for this? Booking the exact same package for 3 adults costs $230 less. Note that per-adult price is also cheaper.

Price Summary

3 Adult(s) x $669.00 = $2007.00
3 x Adult taxes & fees x $389.39 = $1168.17
Total Insurance Cost $0.00
Total Price $3175.17

As a follow-up question, can we buy an adult ticket for a child?

Answer 1

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.