I don't understand your problem about mc behave like toddler have to do with genre isekai from this story? You know that reincarnated genre can get in to isekai genre right? or is this your first time you read manga about mc that get reincarnated to isekai and behave like their age of body they have
99% of the time they don't behave like their age and it's even pointed out by other characters and when they do, is to hide the fact they are actually grown people inside the body of a kid, that's kinda the point of reincarnation isekai... if you make them unwillingly and unconsciously behave their age (like here), makes the whole thing redundant... at that point just make them actual kids, nothing would be lost here if the isekai part was removed, it hasn't contributed to the story in any significant way but for self-insertion purposes.
Imagine other isekai be like "Reborn as a vending machine, I now unconsciously and unwillingly behave like a normal vending machine", it makes the reborn thing even more dumb and redundant, isn't it?