Give this a shot...it's not exact but will give you a pretty good estimate of what you're going to be paying.
http://www.alllaw.com/calculators/Childsupport/florida/
Good luck with it and don't blame the kids for their mom being a jackass. They didn't choose to be put into this position and do not deserve to be treated as if they did. The best thing you can do now is be a good father and work harder to give them what they deserve in life; a chance.
What's done is done now learn from it or you'll make the mistake again.