This is so cute, I'm liking it so far! Usually in isekai and time travel the 'child' became too mature to my liking, I like this version where the isekaid person is still childish and simple just like the age their body is in, if I was isekaid I would still want to become a child again.
(Ah I meant their mindset is still of a child despite having the memories of their past life, I don't mean they're acting childish to fool people to not look suspicious)