Well like the title said, the manga is about a middle aged office worker, meet his mom that had reincarnated. His mom has the previous memory and then after they meet up she's been taking care of him. But his mom is grade schooler now and in her new life, mom got a big sister. What's the title?
Bro... I used mangadex search tag. Used reincarnation and loli tag and got atleast two results that fit your requirements
https://mangadex.org/title/19249/hakka-shoujo https://mangadex.org/title/30333/shougakusei-ga-mama-demo-ii-desu-ka