puppy fat 
noun [mass noun] British
Fat on the body of a baby or child which disappears around adolescence.Smokers get banned, lap dancers told to keep their kit on, Glaswegians told to shun the sunbed, while kids with puppy fat are constantly told they are headed for an early grave....
- It is also his first shot at being a leading man, and bulked out, though not completely free of puppy fat, he acquits himself admirably.
- Their daughter is not just pleasantly plump or carrying a pound or two of excess puppy fat, she is morbidly obese.