how long do they have to wait so they'll stop saying is too soon to have a baby, and try to have one. Do i have to wait until the kids are three?

It depends upon the mom and dad's desire for children. If they both definitely want children, it's usually 2 or 3 game years. It's much longer if either or both of them doesn't want children.

oh, thanks
i guess it does depend, they finally tried having a baby, but no baby came