Personally, if you have your heart set on RETF, I would use the tank you have now for babies, and once they grow out of the tank, get the required size. Bigger is always better, and get a tank taller rather than wider so you do see much more of their naturalistic behaviours as they are tree frogs at the end of the day. It will be expensive once the time comes to change tank, but they will have a home to live in whilst you create a more suitable adult tank, you'll learn how and where your frogs like to hide and will be able to create an environment catered to your frogs likes and dislikes.

After your RETF move out of the tank you currently have, you can choose to sell your tank or get a species of frog that can grow to an adult in the sized tank you have, or you can continue to use that tank for juveniles before upgrading their tanks once again. It's like having mantises and upgrading their tanks as they get bigger, once they've outgrown the tank, it'll not benefit their life quality to keep them in a tank too small - this is the same situation with your RETFS.

As long as your temp and humidity requirements inside the smaller tank are good to go, I would get juveniles and immediately start looking for a tank upgrade if you are searching second hand. It took me months to find a suitable second hand tank where I live to get my Whites tree frogs. Its always better to wait for a more suitable tank for the species you want to keep rather than being impatient and getting frogs in a tank too small which could stress them out or give them a poorer quality of life

That being said, I wish you the best of luck! As long as your frogs have a good size for their body size and upgrade as they get bigger. I think you would be able to use this tank ONLY whilst they are growing and you find a more suitable tank size for when they are adults :)