I've come to the conclusion with my nannie who had triplets it's just time she needs.
If I try giving more hard feed she just gets clumpy poo and she prefers forage and hay,
Same here, I milk my goats and feed the kids by bottle. Two of mine (a BT and a GG) had triplets and are quite skinny. Both get enough food and left with them for about 20minutes (or until they walk away), mix of sugarbeet shreds (soaked), oats and dairy nuts/goat mix. Also tons of freshly cut grass and I have just started to "go branching" most days. As for minerals - Caprivite, supplemented with seaweed powder, garlic and linseeds. Mine also get the first refusal on the weed bucket (thistles, docks, dandelions, sticky willy etc).
They are healthy - and increasing the protein content (i.e. readigrass, alfa a, flaked maize or more dairy nuts) just produces loose stools, as does wet grass.
However if your kids were born February you could wean them off and then milk her (if you want to), if not - letting her dry off will put condition on them in no time.