Hi Emma, and welcome. I love the names LOL. Anyhow, you don't need to get much warmer than 20 degrees C / 70 F for these guys, unless you want them to think it's high summer.

The darker fellow looks alright to me - am I correct in assuming he is eating regularly and not losing weight?

The mouth opening business, how often does this happen, and how many times does the frog do it in one bout? Does it seem obviously distressed? Could it be in the process of shedding its skin when you saw this happen?

One observation on the terrarium: I bet they would appreciate a couple of branches away from the walls in the terrarium, rather than having to sit on the glass. You can use commercial wood, or what I often do for temperate tree frogs is grab some thick sticks from the garden.