What kind of dog? size? Bigger the dog, more mess to clean up. Also, what's the excercise need of the dog?
How committed are you/your sons to taking care of dog. It's gotta be walked, fed, bathed, house trained, taken to the vet....etc.
It's a lot of work. I had a rottweiler. It was a lot of work.
Did I mention it was a lot of work?
People fantasize about all the good aspects of having a pet, but when it's 2 in the morning and the dog needs to go out and pee/shit, and there's 6" snow out, and you can't get the person who orginally wanted the dog in the first place to take the dog out, then you are stuck.
This is a living being. This is not a stuffed animal or some inanimate object. Think of it as another baby/child. Are you ready for another baby/child?