It is very important to note that there is nothing wrong with a Muslim woman breastfeeding a Christian child or a Christian woman breastfeeding a Muslim Child.
In his regard Shaikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states the following: “It is permissible for a Muslim woman to breastfeed a Christian child and it is permissible for a Christian woman to breastfeed a Muslim child, because the basic principle is that it is permitted, and there is no evidence to the contrary. This is a form of kind treatment and Allah has decreed kind treatment in all things. It is proven that the Prophet (peace and blessings be upon him) said: “In every living being there is a reward.”
If breastfeeding takes place, then the Islamic ruling on each of them is not altered because of the breastfeeding. Whoever was a Muslim before breastfeeding remains a Muslim afterwards and whoever was a Christian before breastfeeding likewise remains a Christian.”