¿Estás a amamantarla o a darla leche en polvo?
Sorry, Eric, but that is not correct in Spanish (actually, there are several mistakes). First, "Are you breastfeeding..." is translated almost identically:
There is no construction in Spanish like "¿Estás a + inf.".
Regarding the second part, "dar" requires a direct object (what you give) and an indirect object (the person who gets it). Indirect objects are always "le/les/se", and while direct objects are meant to be "lo/la/los/las/se", some areas use the direct object pronouns where the norm prefers the indirect ones (sometimes for a good reason). In any case, "darla" can only be used if you give her to someone else, as a slave, but the rest doesn't make sense after that.
Dar leche (D.O.) a la niña (I.O.) = Darle leche (a la niña) = Dársela