"My mom entered a contract marriage"
This one is so good and suited for someone who wants child because mc is a genuine child who is suuuuper endearing. She hasnt grown up until the latest chapter.
Also "on the way to meet mom" is just a masterpiece. Mori is so cute even I feel like I want this child in my life. They finished without mori growing up (still as a child)
AH! Sorry! Should've known that you already read both since both are so popular btw I agree so much on the Love so Life one recommended by other reply here but if you love both of the title I mentioned you should've loved that one too!
I didn't mention it before because I thought you are looking for child mc (in love so life, the MC is a teenager but taking care of two child so they appear together almost all the time soo) the theme is nurturing so it will probably nurture your love for children too! (They gave so much toddler parental hacks too!)

Is there a story like this where the MC doesn't grow up to be an adult/teenager quickly, if possible, recommend me one where the MC doesn't grow up to be an adult/teenager. I'm currently feeding my desire to have a child (ToT)