With a Leo that small you will have better luck using the small crickets. They have a natural instinct for hunting moving prey. I never had any luck with even small mealworms with my really young Leos. They just don't move around enough to really grab their attention. Superworms were a different story but they are MUCH more active than mealworms. But even with that, crickets are still the easiest to get them to eat. I would take the mealworms back to Petco explain that they are way too big & get some small 1/4"-1/3" crickets. As small as your Leo is, it is important to get it eating as soon as you can. The T-Rex stuff will be ok as a vitamin supplement, but you also need to get some plain calcium (No added D3 or phosphorus). Calcium is extremely important with young Leos, the last thing you want is a case of MBD (metabolic bone disease).
One thing, you mention having your Leo in a critter keeper (this kinda scares me), how big is it? I've never seen one suitable for a Leo. The under tank heater (UTH) is the correct heat source but I'm guessing you have no way to measure the substrate temperature (floor temp) and with a critter keeper it scares me that it is probably WAY to high, UTH's can get well over 100 degrees. The substrate needs to be between 88-92 degrees on the warm side, you need a thermometer with a probe to measure it. Do you have anything in it for the Leo to hide in? You mention it is just laying there scared. Leos need places to hide, if it doesn't have any then I'm sure it is stressed out like crazy and it won't eat. As small as your baby is, it really needs a humid hide to aid in shedding, it will be shedding about once a week or so.