The main thing to consider when choosing a pet for a child is that, whatever responsibility you hope or expect the child to take for it, in the end it will be a family pet and the ultimate responsibility for its health and welfare rests with the grown-ups!
Even with the most responsible mature child, there will be times (when the child is away on school trips, for example, in the throes of exam stress, or goes off to university) when you will end up having to look after the pet yourself.
With younger or less responsible children, sadly, the novelty of a new pet can wear off remarkably fast and it's only a matter of weeks or months before the parents or guardians are left with the entire responsibility for the care and upkeep of the 'child's' pet.
It's important, then, that as the responsible adult, you also like and enjoy the animal that has been chosen, and can afford to find the space, time and money that may be required to care for it properly.
There's very little point in giving in to pester-power and buying a child a gerbil or rat (even if they have promised to keep it entirely in their bedroom) if you don't like rodents! So, while obviously you will want to take the child's views into consideration, don't feel guilty about allowing your own pet preference to guide the selection process - it's only sensible!