prunes are very good for the digestive system and help to relieve constipation. i am not sure how old your baby is, but you can start adding prune to the milk after your baby completes 6 months. start with very less quantity at first and see how your baby reacts to it.
I read that adding prunes to baby formula or breastmilk is a simple remedy if baby is constipated. Apparently, this is a good way to alleviate constipation if babies who are too young for solid foods. Usually this method can be used for babies 6 months and above.