A slightly different view of the plethora of animals in Children of Men was that humanity had degenerated to the point that people treated the killing of others as no more than slaughtering an animal. There was also the juxtaposition of humans in cages with animals running around on the streets. The value of human and animal life is obviously skewed. There is almost a sense that since humans can no longer reproduce they don’t need another human companion and a pet is superior.
Although I do agree with what you have stated above, there were many scenes where animals were being burned by the dozens. This being said, I think there might have been a hierarchy within the realm of animals. Dogs and cats whom are domesticated are treated as superior and noted as a companion. However, the cows and horses that were burned were not. This parallels the hierarchy within society. A refugee represents the horse and the citizens the domesticated.
I also agree with what you have written above, especially when you say that the value of human and animal life is skewed. However to me it was more that human and animal life became somewhat more equal rather than animals becoming superior to humans. You see animals in cages and humans in cages, as much as you see humans in the streets and animals in the streets. Also, while there are scenes of animals being burned there are also one or two scenes of humans being burned, whether it be to get rid of their bodies or for food ( in the prison).