You may want to post your problem to the expert forum. The information requires some specialized knowledge in pediatrics, post surgery care, medication, etc...however, you shouldn't give supplements, etc. without a doctors recommendation.
One of leaflets associated with the valve may not completely close causing some leakage. It is not uncommon to have some leakage and not problematic.
Did your daughter's medical condition cause myocardial ischemia (decrease of blood/oxygen to heart cells)? If this is true, the condition could cause the muscle tissue that supports the leaflet to dysfunction causing variable degrees of mitral valve leakage.