With the kids for free thing, 2 paying adults need to be in the same room. :
Actually it states that children are free after the first 2
guests pay full price, not necessarily adults. So lets say if I wanted to sail w/o dh and just take our 2 boys, 1 child would cost the same as myself and the other would go free.
Also with this special, you still pay port fees/govt. fees for the "free" kid.
If this is your first cruise, which I'm guessing it is, you really don't want more than 4 in a stateroom anyway.

They are
very small; about 1/3 of a small hotel room. The bathroom is about the size of a non-walk-in closet.
DCL is not anti-family; quite the opposite. Most cruise lines don't hold more than 4 either unless you get into suites. We are a family of 4 and are just about bursting at the seams in a Cat. 9 (slightly larger than the cheaper Cat. 11 & 12 size) I can't imagine adding another child, his/her stuff, and a pack n play to that tiny room. jmho.
Depending on the cruise, it may not really be double the cost. Sometimes it's only a few hundred more. You don't pay per stateroom like a hotel, it's per passenger. One price for the first 2 guests, then a cheaper price for the 3rd and 4the guest all sharing 1 stateroom. The only difference when you need 2 staterooms is that 2 of the children will be paying "adult" prices. Cruise prices don't usually just go by age like WDW tickets do. The 3rd and 4th guests are much cheaper whether they are adults or children.
Since you must have 1 adult officially assigned to each stateroom (they don't really care where you actually sleep) you and child #1 will be paying "First 2 guests" rate. In the other stateroom, your dh and child#2 will pay "First 2 guests" rate. Child #3 will pay "additional guest in a stateroom" rate, which will be much lower than the rest. There are very small surcharges with DCL if the 3rd/4th guests are 13 or older.
I hope I'm not babbling too much. It's waaaay past my bedtime.
As our boys get older/bigger I know soon we will need 2 connecting staterooms anyway. It would be extremely cramped with 2 adults and 2 teens all sharing 1 room.

(1 bathroom, 1 tv)
You will get MUCH more info if you check out the DCL thread here on the dis. It's a very helpful, happy crowd there.
It's not cheap but sooooo worth it. I will say besides showing my kids the Grand Canyon,

a DCL cruise is the ultimate first-class family vacation.

It completely blows away any WDW trip. It just pales in comparison. We are still on cloud nine from our 2nd DCL cruise and can't wait until next Halloween to go again.

I might have to sell a body part but still plan to sail in 2010 too!
Also just to clarify to the pp who stated even infants pay to cruise..... children under 3 are only charged port fee/govt. fees which is really peanuts compared to the rest of the cruise costs. I believe that's a mandatory fee as that is the same as the "free kids" charge.
Oh, 1 more thing... when you book Cat. 12 they assign your stateroom a few weeks prior to the cruise. Your 2 staterooms are not guaranteed to be even remotely near each other. Cat. 11 allows you to choose your stateroom but even 2 together don't connect. If your children are young you'll want connecting rooms which will be Cat. 10 or higher.