@Akikako they've been raiding supplies along the way so I guess we're to assume they managed to grab some boxes of baby formula (and this wouldn't be one of the things people would grab first so more likely they could find it). The mom could have also breast fed along the way to make sure they didn't use the formula until it was needed.
Also I think it is realistic that they would leave the baby behind. The mother felt she had to as her presence would put others at risk especially her baby since his/her immune system isn't fully developed. As painful as the separation would be, as a mother she would prioritize her baby's survival over her own feelings. Now it's true the father could have stayed behind with the baby but I think he couldn't bear to part with the mother. While I'm sure he loves his child, this is the woman he's loved for years and while the others will look after his kid, she'd be left all on her own.