I agree with Wesley to remove them out of the sun. It will heat up the limited amount of water in your lasagna dish too fast and could be part of the reason your tadpoles are not eating.

I fed my tadpoles a mix of fishflakes, algae discs and blanched lettuce. Once they discovered the fish food and algae discs, the lettuce was all but forgotten. I never actually saw them eat and was worried that the food was disolving into the water before they were getting it...until a very wise member said: "Everything poops". Sure enough, there was poop everywhere, and tadpoles don't poop unless they are eating. Relief!!

Are your tads showing any leg growth whatsoever? Once the frog legs pop, tadpoles will stop eating the food you are supplying them.