We domesticated the dog 15,000 years ago and the cat 8,000 years ago. In doing so, we helped create this problem. Now it's our responsibility to solve it. It's also unnatural to be killing so many of them in our pounds and shelters each year. Teach your children instead about humaneness and kindness to all living creatures by educating them about the importance of spaying and neutering.
Fat animals are usually over fed and under exercised.
It's true there can be a tendency for a pet to put on some weight after the operation. But what is not true is that the operation causes the condition. If your pet shows signs of putting on a little weight, reduce the calories and increase the walks or play sessions. That will keep the waistline trim. If neutered young enough, before they develop the habit of spraying, they may never develop the behavior.