It would not make sense (storywise) for Kat/Lea or Rei to have children (by the MC).
Kat/Lea lived next door to your mother, and your mother knew you had sex with both of them, she would know if they have children (Kat is introduced as yout mother's best friend) and it makes no sense that she would hide some of her grandchildren from you and Anna. Kat also knew Jeff's father (she got you the job there after college), so it is very unlikely that there are "hidden" children by those two women.
Rei is (I believe) the architect mentioned in the main game by Samantha as "inspiring you and, in turn, being inspired by you - who is also famous in her own country" Rei is Anna's cousin (their mothers are sisters) and Anna would certainly know if Rei had children by you, it goes against everything that we know about Anna for her to hide that from you.