You need to find a way to monitor your temps and humidity. Without that info, there is not a whole lot of help we can give you.

Are the pebbles and gravel small enough that they could ingest them? White's tree frogs are ravenous eaters and could easily scoop up pebbles, causing impaction and stopping them from eating anymore. I would get some kind of substrate like plantation soil or coconut husk fiber and get rid of all pebbles and gravel. You could also put live plants in there to help level out the humidity.

Do you have a screen top? Is there good ventilation?

I'll be waiting on pictures.

In the meantime, you may want to read this --- Frog Forum - White's Tree Frog Care - Litoria caerulea --- to learn how to provide them with optimal care and prevent stress that will lead to illness.

ETA: With plants and a substrate that they can grow in, you'll be doing less cleaning too. I keep isopods and springtails in my tank and they do most of the cleaning for me. I've only changed the substrate in my gray tree frog tank twice since building it nearly 3 years ago. I have a drainage system in the bottom so that I can pump out any extra water and prevent my plants from rotting.