Welcome aboard. Ok first problem I see is the water. You should never use distilled water in their water bowls, it just to clean and cannot provide trace elements needed by the frogs. It is very good for misting though. Instead fill their bowl with dechlorinated tap water. You can use any brand aquarium dechlorinating solution.

Second, mealworms are not good in this situation. In my experience, red-eyed leaf frogs, Agalychnis callidryas ignore them. They are just not active enough to get their attention. The crickets will work, as will flightless fruit flies and bean weevils providing the frogs are small enough (adults will most likely ignore ff's and weevils). When dusting with vitamin and calcium powders, it is a good idea to feed just before the lights go out for the night. To do it earlier, gives the feeder insect time to clean itself before the frogs wake up.

A third possibility, could be the enclosure itself. What is it made out of? I am a breeder of Agalychnis callidryas myself and I had been losing froglets over the past few months. I was dumb-founded as to the reason why, but after talking to a much more experienced breeder, I found out it could be the cage itself that was killing them. She had told me that she was losing them at one point because the frogs' skin secretions were breaking down the poly-carbonate wall of the enclosure they were in. Faunariums/Critter keepers are made of poly-carbonate. So I switched, them to a small glass tank. The dying seems to have stopped and I am seeing some signs of improvement, like frog droppings. At the height of the die off the amount of droppings had, well, dropped off. Of course if your enclosure is glass, well I just wasted a lot your time and mine typing this.