Thank you for your query,
You did not mention the age of your niece?
As in early infancy kid will pass stool once in 2 to 3 days and it is not considered constipation
Constipation will be aggravated if it is present by artificial and other feeds.
Exclusive breast feeding till the age of 6 months will help the child to pass soft paste like stools and infants on breastfeeding
don't have constipation.
Ask her mother to take balanced diet and an extra 500 kilo-calories diet for the proper production of milk.
Hope I have answered your query, if you have any doubts please let me know.