“My children want a Puppy for Christmas. The truth is I do not want to be lumbered with looking after another being and I am not sure that they will want the responsibilities that come with Puppies!”
The saying is A Dog is for life and not just for Christmas! Get your kids one of these stickers and explain your thoughts and feelings. Perhaps you can puppy / dog sit a friends dog for a week or two (perhaps when they head off for holidays) and ensure the children do everything that having a dog entails eg feeding, washing, walking and poop cleaning! I think after a couple of weeks, the kids would lose interest unless they are really ready.
An alternative is getting a cyber pet. Places like The Gadget Shop or I want one of those Ltd sell them.