Dear friend, it depends on what repellant you are using. There will be instructions provided along with the repellant or you can find it on the tube. pls read the instructions regarding the age group which you can use it on. If it says safe for infants or babies you can use it. Else better to use mosquito nets treated with repellants rather than applying directly on the skin of the baby. Also keep your environment clean and stagnant water free to reduce the mosquito population.