Hey Nik,
I remove frog-lets from their tadpole rearing jars as soon as they pop their front legs. Move them to a container or tank with shallow water at one end and an elevated portion on the other end with damp sphagnum moss on the elevated portion. You can take gravel and make a slope so the the frogs can walk out of the water and return to soak if they need. It sounds as if they are drowning as Heather stated. I use a high protien fish food (Omega One) for tadpole food as well, its higher in protien than lettuce. For the frog-lets i feed springtails, fruit flies, and dwarf isopodes and rotate dustings with Repashy calcium plus, Rep-Cal Calcium, and Rep-Cal Herptivite. Good luck, keep us posted.