Hi welcome to Health care magic..........
Get her to a pediatrician and get her evaluated for constipation
1) medical conditions like hypothyroidism
can cause contipation which needs to be ruled out , because it affects childs growth and development.
2) if no other causes of constipation is detected , then she can be put on dietary management
Plenty of water , fruits and vegetables.
Avoid too much of choclates and fat rich foods .
if this does not work then Syrup Lactulose
can be given in increasing doses begining from 3 ml .
BUT but but before starting on Lactulose please consult a pediatrician
and rule out other causes of constipation