Great cattle for small acreage! You might be able to find some unregistered Dexters, but if you ever plan to sell calves (other than for beef), most buyers will probably want an animal that can be registered and that's easier if you keep up with the paperwork.
You can probably expect to pay more for polled or dehorned animals, as well as those that are halter-broken and able to be lead (usually people who want their children involved want to avoid the horns). Once they've learned to lead, etc., they don't forget, but you will need to work with them some. Dexters are smaller than most other breeds, but they are still cattle, and not always the ideal "pet" for young kids.
You can contact your ADCA regional director (you can find the listing at
http://www.dextercattle.org) who will be able to point you toward some breeders in your area; you can also click on the "pedigree finder" on that site to locate Dexter people near you. Visit as many as you can before you purchase because you learn a lot with each visit to a farm. You might want to consider two heifer or steer weanlings (not bottle babies) as cattle need company of their own kind to be content.
Depending on what your acreage has growing on it, plan to supplement with hay (esp. in winter or during dry periods), fresh water at all times and minerals; some people give grain, others don't; also need to provide shade in summer, a shelter or windbreak for winter (ours seldom use it, but it's there if they want it and the calves seem to like it).
Hope this helps...