Prairie, I'll expand on the "file for child support now!" comment.
Generally, child support is figured on a percentage of the father's salary. I don't believe the children he's living with count in that figure (or not that much from what I've heard). Usually it's just a standard percentage of his gross salary based on a sliding scale.
HOWEVER, the first person in line filing for child support gets the biggest piece of the pie - because his ENTIRE salary is used when they configure what percentage of it will constitute the monthly child support payment.
Subsequent child support orders are figured on his salary AFTER the first child support amount is deducted from the gross - so the 2nd (and subsequent) child support orders are figured on a lesser portion of his salary and are lesser payments than the first order. That's why it's in your best interests to be the FIRST child support order!!!
Talk to your lawyer; I've read of quite a few couples 'separating' and the wife filing a child support order for the children of the marriage so that 2nd (and subsequent) child support orders were configured at much lower rates.
It may sound cruel to the OW's child, but your husband's careless stupidity shouldn't compromise your own children's health and well-being because a big portion of your family budget is now going to support his child. Why should YOUR kids go without because of your husband's irresponsibility?
And Prairie, unfortunately the chances of this having just been a one-night stand are just about nil. Cheaters always lie and claim it was a one-night stand - I think just about every cheater has tried that lie when they got caught. I don't think you nearly have the whole - and real - story on these two, just yet. Right now, it's more important that you make sure that YOUR kids are provided for and let your husband get a 2nd job if he has to. Too bad, so sad for him if he has to.
I'd get to a lawyer immediately - before the DNA testing. Hey, if the testing proves it's not his kid, then great - your 'separation' can be dissolved and your child support order can be cancelled. But if it IS his kid, then at least you've done all you can to provide for YOUR kids first.