The kitten my fella came home with was also too young to be away from her mother. She didn't gorge but rather, was too easily distracted to eat.
There's a type of nursing milk replacement called KMR and if you have a Petsmart, they sell it there. You can get the pre-mixed in cans, or powder to reconstitute as milk. And as you also probably know, cow's milk isn't a good choice.
Since the kitties are already eating solid food, you may not have success in getting them to simply lap the KMR milk and may have to mix it. I used kitten food and mixed a teaspoon of KMR powder, water and canned food together to make a slurry and Simone the kitten would eat that. I gave her 4 small meals over the course of the day. First thing getting up, once again before leaving work, when I got home and again before bed.
If your kitties aren't used to canned food, they may not want to eat it. You may want to tempt them by mixing in Gerber's chicken baby food, or Gerbers beef baby food. Make sure it's just chicken/water or beef/water and avoid those that have onion or garlic in it. You probably already know this, but these things can be toxic to kitties. It bears mentioning, just in case though.
For some reason, the baby food is wildly appealing to cats and I keep some on hand for all of them to offer as a treat and to use if they need medicine that can be disguised in the food.
My kitten lost interest in the kitten canned food early, but will eat Wellness brand kitten dry. She gets adult canned food now (she's just now 5 months) plus the Wellness kitten dry.
The Wellness is very rich though so it's good to mix it with the current food, gradually increasing the ratio of the new food with the old food so the kittens don't get diarrhea.
Canned foods that you might try first Nutro kitten, and then some all stage canned like Felidae or Wellness if they don't want the kitten canned.
I'd probably avoid Friskies and Fancy Feast canned kitten or adult if possible. Whiskas, Sheba, 9 Lives, Paws are at the low end too but not necessarily on price.
And even though your kittens are young, who knows what would have happened to them had you not come along? People who get rid of kittens at young ages probably would simply get rid of them if someone didn't adopt them, so you've done a good thing. Good luck with the KMR!