Book the room in the normal fashion, X adults (18+), X juniors (10-17), X children (3-9), one infant (0-2). Reserve Magical Express in the same manner.
If they jump to conclusions (or lose some of the information in the translation) and send only four vouchers, then call DME directly. If the additional voucher does not arrive in time just show up at the welcoming area and they will either print a fifth voucher or let you carry the infant on board without. (I'm not sure how the subcontractor Mears counts heads and that determines which of the preceding steps they take.)
If you need more yellow baggage tags, they'll send more on request without regard to the number of people.
Having a copy of the birth certificate makes things go more smoothly although there have been no printed requirements other than for the free-admission-on-your-birthday promotion done a few years ago.
Magical Express hot line number: toll free 866-599-0951
Don't forget, the 2yo does not need a park ticket or fast passes to accompany you but he does count when reserving times at restaurants (no charge at buffets or if you don't order him anything).
