Unfortunately, your illegal migrant situation has put you and your family in a difficult situation.
Although you're married to an Indian citizen + you have lived in India for 7+ years (the requirement to apply for citizenship), you cannot apply for Indian citizenship by marriage because you're an illegal migrant.
Also, your baby is not eligible for Indian citizenship because one parent has Indian citizenship but the other parent is an illegal migrant. Hopefully, you have registered your baby's birth in India.
I empathize with you and your family. IMO, in the best interests of your baby (who is stateless), you and your family may want to reconsider returning to Sri Lanka. Do try and get official documents for all of you when you return to SL.... and then consider moving to another country