Funny one here. If I have a child with my girlfriend then marry her the kid becomes "mine". What about any kids she had before??? Also if we divorce what happens to the standings of the kids then?

By marrying the mother you legitimize the child.

As the other children are not yours, you do not legitimize them. Ask about this at the amphur, you might have an option to legalize them as yours or not. (If the mother was already legally married when the children were born they already have a legal father anyway).

Upon divorce your child will still be your child, with all rights and responsibilities (including to take care of the child financially). Regarding the other children no such obligation exists, legally they are not yours.