I get asked this sort of question all the time. It's along the same lines as, "Should my school buy netbooks or laptops?" "Laptops or desktops?" "Desktops or thin clients?" It is, however, a remarkably difficult question to answer without quite a bit of background knowledge.
A major part of our job as IT professionals is defining requirements. This isn't something the end user is expected to know how to do, any more than I would know how to install a new furnace in my house. This is why they keep us around and why plumbers make more money than I ever will.
Sticking with the furnace analogy, the same principals actually apply to working with my plumber. A few years back, I built a house (by built, I mean that I did the general contracting; handy I am not). I knew that I needed to heat the house, but I didn't have any idea what was involved, aside from some reading that suggested that a hydronic system made the most sense here in New England. So I called a reputable plumber who promptly started asking me lots of questions, largely avoiding any technical questions (that I either wouldn't understand or wouldn't have the answers to), but ultimately helped him define what I needed to appropriately heat my home.
By the time we were done, I had a system that could easily be expanded if I ever wanted to heat my driveway (did you know that you can heat a driveway to melt the snow, no shoveling required?), maximized efficiency, fit easily around furniture, and was optimally sized for the house. Our discussions (and his inspections, measurements, and calculations) went far beyond me telling him that I needed to heat my house.
Along these lines, here's an email I sent to a teacher who asked for recommendations about laptop carts. Her inquiry was very typical of the sorts of questions I'm asked all the time. As many of us know, actually defining requirements can be the hardest part of a project; implementation is just labor.
It really depends on the purpose/usage model you have in mind. Will these mostly be for web access and light productivity applications? Or do you expect the older kids to be creating content like video or audio? What are you giving up in order to pay for them and, along those lines, what is your budget?
While any initiative that gets kids using computers early is probably worthwhile, we've certainly seen that Macbook laptop carts are largely wasted on the younger kids. There is a lot to be said for netbooks in this space, especially Classmates that strike a nice balance between price, interesting function, and a form factor that's comfortable for K-8.
That doesn't include any software that you might want to install (Microsoft Office, for example, or educational software you might already be using). They come with the wide variety of software described here: http://blogs.intel.com/technology/2009/08/classmate_pc_as_a_one-to-one_l.php
This also doesn't include your admin's time. They need to be set up, maintained, re-imaged, etc (this goes for any set of laptops/netbooks, not just the Classmates). If you want them online, you need to ask how they will connect. Do you already have a wireless infrastructure or would you want a cart that included wireless? If the latter is the case, do you have a network connection in every room?
In some cases, a stationary, wired computer lab (or server with thin clients) can serve a variety of needs better. We've seen difficulties in younger kids just getting the laptops out of the cart, opening them up, turning them on, logging in, etc. By the time a kindergartner does all that, it's time to put them away. They lack these difficulties in a stationary lab.
Similarly, in some settings, classes can be served as well or better (depending on the teachers) by installing a few machines in each classroom (whether netbooks, laptops, or desktops) to allow the instructors to differentiate.
I'm sorry I've answered your question with lots more questions, but they're important to the success of the project (and to even picking the right project for your context).
Let me know if you'd like to have a further dialog - email is great and my phone number is below. I'd also be happy to help you and your admin define requirements and price out an appropriate system. I'd probably turn it into a blog (as I will with this email) since your questions are timely, frequently asked, and point to some of the real challenges in meaningfully rolling out tech to your students.