I've traveled around Mauritius a couple of times, but not recently. There are/were a few shopping malls with what I would call European-style supermarkets and drug stores, but I am not sure whether they would have survived 2 seasons of Covid/no tourists.
Are you staying in a big hotel? They often have a shop for guests needing basic medication. I expect you could chat to reception and get some sensible advice from them or guest relations.
Otherwise, I recommend asking for advice on Tripadvisor's Mauritius forum. There are a few expat Brits and South Africans on there that (used to?) split their time. They will have a better idea of what is going on.
October is at the tail end of the warm, dry season (it gets hotter and wetter after that). I recommend you talk to a specialist travel clinic because a very young baby won't have had any(?) vaccinations. Tetanus, diphtheria, polio and hepatitis are recommended (less important if you are in a 5 star hotel, more so if you are more rural). I get bitten out there (but there isn't malaria). Sort out a bed net for the baby.
Otherwise clothing is going to depend on whether your rooms are in the 30's or low 20's (aircon?). Watch out for the reflective nature of the sea and (usually). white sand if you go to a beach.
Otherwise pack a thermometer and calpol and some phone numbers for local doctors/pharmacies ( hopefully you won't need them).
Pack more nappies than you can possibly need for carry on in case you get held up somewhere. Little babies have little bladders!