We have a 9year old as our 9th guest. She does count as being there but she doesn't count for the $100 if she was 10 she would count and cost $100. I'm not sure what the deal is for 3 and under my guess would be they wouldn't count as they don't pay admission for the parks but I'm only guessing.
i know disney recently changed their intimate packages to include upto 20 guests. i would check with your sales manager or event manager/wedding planner at dftw about kids counting or not now.
I have confirmed with my EM that everyone counts, however, there is no additional ($100.00) fees for guest counts up to 20. You will have to purchase additional beverages for the champagne reception.