Food, a litter box and a few shots every few years and a cat will tend to take care of itself. This is good for a kid that may not be able to handle the responsibility of a dog (which will require more attention). There is no need to take them outside, or really wash them all that much either. A hands off animal for the most part.
Food for cats generally costs less and they eat less of it compared to a dog. So when it comes to financial concerns they generally cost less over the years, illness withstanding.
It may be easy for a child to get scratched from a cat that is not declawed (which most vets recommend not to do as it literally is cutting off their fingertips at the bone), which may also cause infection due to them digging in a litterbox when they go to the bathroom.
With many different breeds of dogs there is sure likely to be one that will fit your living arrangements and personality. This way you can match up to a species that is lax and lazy or energetic and playful and anything in between.
Unlike a lot of cats who can often be aloof, dogs like attention and want to play a lot. This means going for walks, running around outside, playing fetch and teaching them tricks.
A dog will need walked or at least taken outside multiple times a day. If a child is being taught to take care of a dog that means the child will be going outside as well.
Purchasing a dog can cost quite a bit more than a cat (depending on breeds) and then also generally eat more food. They may also require more vet visits. All in all a dog often costs overall quite a bit more than a cat.
Dogs need to be taken out multiple times a day, they often need more vet visits (than a cat) and usually require more attention as they are social animals.
All behaviour is dependent on the influences of inheritance, learning and the environment. Aggression may be normal or abnormal depending on the context. It is important for dog owners to be aware that aggression can be caused by underlying fear and anxiety.