A mother's milk contains antibodies to viruses they have encountered in their life. A baby's immune system is tuned to pick up on those antibodies and make its own memory cells to bootstrap an innate set of those antibodies generated from the babies matured B cells.

RNA does not alter DNA unless it has code for a machine called reverse transcriptase that can write the RNA into the cell's DNA. There are viruses that do this like HPV and HIV - but coronavirus is not one. The problem with these kinds of viruses, though - is that when they write a virus to the DNA - its not selective as to where so they can write it to a location in the cell that can disrupt apoptosis ( programmatic cell death ),