Goat lady here - 100 head breeder of Boer goats.
Why isn't the nanny raising her own baby? If she is nursing her baby, then the mommy is who you need to feed - buy a bag of Purina Goat Pellet and feed the mom and offer it to junior.
Otherwise if you are feeding, baby goats at that age should be getting several ounces of goat milk replacer (from the feed store) fed in a bottle at several times a day. Goats are ruminants. They need to be all-day grazers so simulating this is a great start. If mommy is feeding the baby, let her do her thing. The baby will look like it never nurses much - maybe one or two sucks ever 5 minutes or so and then maybe some lengthy nursing and then two hours without. Mom and baby are taking care of each other just fine at this point - so don't let them fool you into thinking baby isn't getting much. I've bottle raised more goats than I can count. It is so much easier to let mom do the work, and just feed mom a good quality feed. Don't forget the dewormer and the CD&T vaccs!! Scour meds work well if the whole milk causes diarrhea. Diarrhea can also cause low blood sugar and weakness and dehydration. There's quite a lot of work in nursing baby goats - if mom is capable, then stand back and let her do all the dirty work. A good, medicated pelleted feed will do wonders for keeping them both healthy.