If your child is constantly falling sick there is a mantra from the Durga Saptashati in the Markandeya Purana which will cure sickness of child.
The mantra should be chanted after taking bath in hot water in the morning hours. The mantra should be chanted by the mother or father or by the child.
The child should be always be clean and should eat good food before chanting the mantra. The room used by the child should have good ventilation. All unwanted clothes and old things should be kept away from the house.
Mantra To Cure Sickness Of Child
रोगानशेषानपहंसि तुष्टा रुष्टा तु कामान् सकलानभीष्टान्।
त्वामाश्रितानां न विपन्नराणां त्वामाश्रिता ह्याश्रयतां प्रयान्ति॥
Rogaansheshanpahamsi Tushta Rushta Thu Kaman Sankalanbhishtan
Tvamasritanam Na Vipanranam Tvamasritha Hayasrreyantam Prayaniti
रोगानशेषानपहंसि तुष्टा रुष्टा तु कामान् सकलानभीष्टान्।
त्वामाश्रितानां न विपन्नराणां त्वामाश्रिता ह्याश्रयतां प्रयान्ति॥
Rogaansheshanpahamsi Tushta Rushta Thu Kaman Sankalanbhishtan
Tvamasritanam Na Vipanranam Tvamasritha Hayasrreyantam Prayaniti