A google search will reveal that for a long time there has been something going on with hotel room pricing as a function of occupancy in Dubai, where adding a second child causes prices to skyrocket. There was even a newspaper article written about it...back in 2016. But none of this discussion even suggests a root cause. Interesting.

For this particular poster, if they wish to stay within the rules...have they considered looking into Hilton's connecting room booking functionality (1 adult and 1 child per each of two rooms)?