Well, first let me answer a question with a question. Would a person paying child support want to increase their own child support? Probably not, so the MFA really only applies to paying parties. Now, as to which kids can be included, I agree with you that using all the children on both sides would be the best way.
What I'm trying to say is that just because Mom births another child does not mean you can walk into the court room and ask for a reduction in child support. see "in defense of a requested child support increase." Also, just because you have a new child in your family, it doesn't mean you can go ask for a reduction. Heck, you could have triplets next year, which would drastically change things, but the way it's written, you cannot automatically request a reduction.
I'm not saying I agree with it, but that's how its written.
In the future, could you please offer some more details on your questions and your case specifically so I can give you a more direct answer? Usually its the case where Dad wants to ask for a reduction in support because there are new children in the family. Honestly, I think that's how it should work, but it doesn't work quite like that.