If baby below 1 year, best way is breastfeeding to boost immunity. If any issue with breastfeeding, then choose Prebiotics rich FM.

If baby is below age 1 please ask mother to continue breastfeeding as that is the main source which would improve immunity

Encourage your cousin to continue with breastfeeding. It is the best way too boost immunity

Check with aptmil formula milk with prebiotics in it which helps to keep the gut healthy and build strong immune system