Good move on adding the Under tank heater! With that on one side of the tank, the babies should start doing better almost immediately. Can you check the actual temperature on the bottom of the tank, where the gecko's bellies will be?
Also, are you feeding in the evening? Because they are nocturnal by nature, the babies may be more inclined to eat if you feed right after sundown. Mealworms move around more when you first put them in, making them more likely to be eaten when first added. A few of my leopard geckos, mostly younger, need to be tempted a little before they'll eat. I dangle one mealworm in forceps (aka hemostat). It wriggles a lot, hopefully catching the leos attention. Then I can lead the baby/babies to the food dish.
Another thing to try is a food dish with shorter walls. Until they learn to associate feeding dish with actual food, some act like its just a weird piece of cage furniture.
Other than that, all I can suggest is things to lower general stress levels .. being sure there are plenty of hides on the warm side and a couple on the cool side, large enough moist hide, cage not to huge for the babies to find their food in, cage in a relatively quiet place -- not in the living room if you (and your family) watch much loud TV especially "action" shows.
Also how large are your babies? Full grown mealworms can be pretty darn huge to a hatchling! Full grown crickets can be impossible for small leos too. Can you get 4-5 week old crickets? They're about half an inch long and young leos (mine at least) LOVE chasing & eating them. hm.. . that's all I'm coming up with off the top.
I didn't read your earlier post, so please excuse any redundancy. Best of luck to you & your critters!
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Andrea A. 