A puppy is a living, thinking, feeling being that did not ask to be here, but does ask to be loved and cared for. This does not mean a bowl of cheap kibble once a day, with a cursory pat on the head. What it does mean is a well-balanced diet, regular vaccinations, brushing, toenail clipping, ear cleaning, dental care, adequate exercise, training, shelter from the elements, and - oh, yes - lots and lots of love, each and every day. A puppy is a lifetime commitment, and that means for its entire lifetime.
A toy, to amuse your children for a few weeks or until the novelty wears off. A puppy is a child, itself, which, as with all children, requires a great deal of attention and training to become a decent adult in future years. If your children are toddlers, they can inflict unintended tortures on a young puppy that could permanently scar its personality and behavior.
A teaching aid, to be used to instill a sense of responsibility in older children. Unless your child already has the maturity to perform regular chores, it is unfair to your puppy to put its entire well-being into the hands of your child, alone. The child/dog relationship's greatest value lies in the special camaraderie and unconditional love that exists between them. Forced responsibilities can result in resentment. If this happens, you will probably end up doing the chores, and the dog could end up ignored and unhappy.
A burglar alarm, to chain in your yard or leave unattended to bark at all hours of the day and night. This is not a crime deterrent; it is a public nuisance that will at least make you unpopular with the neighbors, and might result in costly fines and civil penalties. Even worse, a dog on a chain might be at the mercy of teasing children and aggressive strays. This, in turn, can make your dog aggressive. If the dog then breaks its chain or digs out of the yard, you could end up in court if a neighbor's child or pet is mauled or killed.
A gift, unless the giver is very sure that a specific puppy is wanted. The mistake of selecting a puppy poorly-suited to a recipient's likes, personality, or lifestyle can end up causing both the recipient and the puppy nothing but heartache. If you know that someone really wants a puppy, consider instead getting them a gift certificate for items at the pet-supply store, for vaccinations, for obedience classes, for their favorite dog food, or for a sitting with a good pet photographer.
A fad, in the same vein as a pet rock or a lava lamp, to be put in the closet or sold at a garage sale or turned in at a shelter when the fad passes. The dog will still need to be fed, cared for, and picked up after even when the breed is no longer in vogue.