Please consult a PD. Just to share my lo is 2.5 years old and his weight is 11 kg and 84cm. He eats and drinks well. My PD said is fine as his growth and weight has been at 10th percentile since 6 months old. So his physique is small. But as long as he is healthy, eats and drinks well, it's fine.
Give her vitamins supplement. Or better consult a pedia ..
how is her health, does she fall sick often or not?