I don't know in which part they're acting like children except when they're trying to. They're really mature for their body age and if it's because of them crying then i think it's reasonable. Living for 5 years confined and being abused even if you're 20 year olds, that's still more than enough to leave a scar and trauma. Beside, the isekai part isn't like they're being reborn into a novel so talking about their past live isn't necessary since their character development is already being shown enough and should i say almost realistically by the author. Ofc it won't be that easy for real people to get pass their trauma but the author did a great job to try making it convincing.
Why even add that they were isekaied of you’re not going to mention it after the first chapter and have them act like children...