I have 2year old don’t won’t to eat and a 4week old constipated

My son is 2 years old he don’t won’t to eat and my other son is 4week old constipated he didn’t poop in three days and leave with my mom and she worried that he have pooping and breastfeeding and giving him formula I’m not eating enough each day what can I eat while breastfeeding that wont constipated my weeks old