having sex with a girl who has a kid

All the sarcasm aside, they go back to the way they were.
That's how it works, everything you change wants to go back to the normal way it was.
Like braces, you have to wear a retainer for....a long time because your teeth naturally want to go back the way they were.
Or that stubborn hair that you try to part with the rest of your hair that keeps going back.
Anyways, you have a baby, it takes a while for the vagina to heal up.
But it goes back to the tightness it was before.

 

You nailed it, what she ^ said