My suggestion is don't! My kids are off the charts tall and off the charts underweight. They are perfectly healthy, thriving and strong. They do not need to be on the charts, their bodies are smart and they will grow in accord with their personal genetic code.
Fattening a child up now will lead to terrible eating habits as an adult, higher body fat and a slew of health problems. My mom, btw, is a clinical dietitian and she's the one who has explained this to me. I've also studied nutrition in college, but not as much as her (She has two master's degrees in the subject). I am certain that unless a child has developmental issues due to malnutrition, it is unwise to fatten them up.
We only develop new fat cells as children. They tend to expand and store more fat as adults. There are hormonal issues at play here, too complex for this post, that drive people to eat more if they have more fat cells. That's not even mentioning the fact that when you fatten a child up you destroy the natural mechanism that tells a kid to stop eating when their bodies have had enough nourishment.
Your granddaughter's body is a lot smarter than any conscious thought about what she "should" look like. Trust it.
If, however, she is not thriving developmentally then talk to a pediactric nutritionist.
All the best