It's OK, I understand that it is like 5 times harder to take care of young frogs because feeding is often quite difficult! I hope they had decent lighting though! If you had rocks for the little guy to sit on they should be fine as long as they can actually climb onto it; your picture makes it look evident they can. Tree frogs drown fairly easily in comparison to some species and I've often found them drowned in rain barrels or steep garden ponds (made of plastic), because once their sticky feet become too wet they keep on slipping down and continually make the sides even more wet and drown after several hours. I see you have a rock so I honestly don't know what happened. Raising tadpoles is a great experience but once metamorphosis is complete it is safest to release them back into the wild. Raising a mature adult frog is the easiest! Don't feel any guilt over what happened it is all about learning from your mistakes!