No scholar has said that breastfeeding a child is one of the things that invalidate wudoo’. The things that invalidate wudoo’ are defined and limited, and have been explained in the answer to question no. 14321.
The scholars are unanimously agreed that anything that comes out of a site other than the front or back passages does not invalidate wudoo’ if it is taahir (pure), but they differed concerning that which comes out of the body if it is naajis (impure), as to whether it invalidates wudoo’ or not.
This has been discussed in detail in the answer to question no. 45666.
Al-Mawsoo’ah al-Fiqhiyyah (17/111).