I was reading Ikumen After and it made me think of another manga with a similar theme.

The uke/seme has a son with his previous partner but he doesn't see him often as I think the mum has custody. One day, dad went to pick up his son from school and the mum also arrived at the same time. The son ran over to his mum first and I think it made dad quite sad that he picked mum first.

I remember one panel where the child (who's somewhere between 5-10 y/o) is standing next to his mom, but he only reached up to her knees. It stuck in my mind how disproportionate it was ..