I read that as 'a main character with a kid's intellect'.... and here I had several hundred recs for you.. but reading the fine print... sigh
Youjo Senki This one should supossedly fit your bill haven’t started reading it myself yet. Sadly most smart kids are simply reincarnated adults in kids bodies on here
Manga wo Yomeru Ore ga Sekai Saikyou ~Yometachi to Sugosu Ki mama na Seikatsu~ I think the MC is pretty young (~6years?)...