After much research, I just recently made the decision to buy and am in the process of closing on a resale. Here's some conclusions I reached based on the research I've done (hopefully people with real-world experience can comment on the validity of all of this):
Negatives
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1. DVC is one of the more (probably the most) expensive timeshare plans. I feel comfortable making this statement, because I spent a LOT of time analyzing different plans.
2. Due to maintenance fees, with any timeshare you're locked into spending money every year on vacation whether you want to, or can afford to, or not. With 150 points, you'll be spending about $600 every year on maintenance fees. If that might be a problem, think twice.
3. All timeshare plans require significant planning to use effectively, compared to not having a timeshare. In order to stay exactly where you want to stay, you have to make plans far in advance. Last minute cancellations can result in a significant loss of timeshare value. You have to be sure to do any banking of points before the deadline.
Positives
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1. DVC is the only timeshare plan that lets you stay at Disney. Even though Interval International lists DVC resorts in their directory, it's next to impossible to trade into DVC (based on discussions I read on several timeshare message boards). And although many timeshares have locations close to Disney, we know better, right?
2. If you're definitely going to be spending money on a Disney vacation every year, and staying at the higher-end resorts, DVC is the cheapest way to do it.
3. DVC is one of the easiest and most flexible timeshare plans. For example, booking at the 11 month window, and probably up to just before the 7 month window, you have a very good chance of getting your home resort. It also sounds like the wait list works very well for most reservations within the 7 month window. Compare this with RCI, for example, where people talk about making reservations 2 years in advance to get what they want. Also, most other timeshares are geared toward 7-day vacations and often allow check-in on only one day of the week, usually Fri-Sun. Even other point systems aren't flexible with less-than-7-day reservations (with Worldmark, for example, you can do this only within 60 days of your check-in date). With DVC, you can check in when you want, for how many days you went, with no restrictions.
4. Having a kitchen in the room at Disney is going to be awesome. This is actually one of the biggest things that tipped me to the buy side. With a standard room, we almost never make it to early entry. Having breakast at the restaurants takes so long by the time you get seated, get your food, and pay the bill. We just never manage to get ourselves and our daughter up and dressed in time. Now that we'll be able to make our own breakfast in our room, you can look for us at the front of the early-entry lines
5. 2-bedroom rooms are great for travelling with someone else. My daughter loves sharing space with her cousins or friends when we travel. But it's not always easy to get connecting "regular" rooms. With the 2-bedroom, my daughter (5-years old) gets the thrill of having a "sleepover" without the anxiety of being too far from Mommy and Daddy.
Not to take anything away from Caskbill's previously mentioned DVC planner, there's also an online planner right here. Just click "DVC Points Calculator" at the top of this forum to play with different rooms and dates to see how many points you need.
If the up-front money is an issue, one option is to buy a 100 point resale (I currently see 5 such contracts available on the various resale sites). Using Wilderness Lodge as an example, in "Magic Season" (next-to-highest season), you could stay in a studio every year for 5 nights, Saturday through Wednesday. Or you could stay every other year in a 1-bedroom for the same 5 nights. Many of the debates about whether to buy into DVC or not assume an "all or nothing" scenario (you either always use DVC or you never do). But just because you go to Disney every year doesn't mean you have to buy enough points to use DVC every year. Some years, discounts will be more readily available and you can bank your points and pay cash those years. You can also make fewer points work by splitting a vacation between DVC and non-DVC resorts. This is quite easy since, as other threads have mentioned, Disney will move your bags to another Disney resort for you. Animal Kingdom Lodge makes this work really well. Since the Animal Kingdom Park is kind of off on its own, staying at the lodge there for a couple of nights can cut down on shuttling around Disney. And there's nothing like having a Savannah View room for a couple of nights so that when the kids wake up in the morning they can look at the animals through the window or from the balcony instead of pulling you out of bed to get to a park. Early entry is great, but some mornings you just want to sleep in

In any event, when you later start needing a 2-bedroom, you could buy another 50-100 point resale (or add on through Disney).
On the other hand, if there's any way to swing a larger point contract, it can easily make sense to buy more points now than you actually need. You already stated, as many other threads have pointed out, that you will most likely end up needing more points as the kids get older. And a few years from now, the cost of points is more likely to be higher than lower. More points can also let you defray the cost of annual maintenance fees by taking someone to Disney with you and charging them a reasonable rate for the room. Some people don't like the idea of charging family members for DVC reservations. But in my case, my parents have stated they'd be more than happy to share a 2-bedroom and pay us $100 per night. Even though I'll probably charge them quite a bit less than that, it will still help to make the annual cost of vacations cheaper. And I have other relatives and friends eager to pay me for their own trips.
If you're not in a hurry to buy into DVC, you could also try to save a few hundred dollars buy bidding on some resales at a couple of bucks below the list price. Now seems to be a good time to do this. Several weeks ago, Disney bought a contract that I bid $72/point on. But the one I'm closing on now I got for $69/point. And other posts indicate resales recently going through for around $68 or $69 per point. If you're not in a hurry, you can afford to bid low and have Disney buy the contract a couple of times. Sooner or later Disney is bound to let a cheap one slip through.