imac until high school...
Yikes, the thought of a 5-7 year old running around with a laptop doesn't end well. My eldest started playing around on an iMac around 5, and my younger one was around 3. Fast forward a bunch of years and it became clear sharing computers among the brood was NOT going to work, even when they grabbed my laptop we were at least one computer short. Once they hit high school a computer, and arguably a laptop, is not really a luxury item, it is a necessity.
Buy a refurb, that is what I did, and the only thing different from a new box is the box itself (it is a generic brown box, as opposed to the Apple white/black boxes. Big deal.) You get the same warranty protection as new and save some bucks. GET APPLECARE. Buy it on Amazon, or Ebay and save a few bucks, but get it. Get sufficient ram for now, and also 2-3 years into the future. 4gb is probably a good amount. Hard drive space is not a big deal, as you can easily add an external HD when you need it. Prices for drives only go down, storage capacity only goes up.
Put it in a common area, not their bedroom. Unfortunately when they are older and have laptops you will not have too much control. Only realize by then they have complete privacy by their internal world of communication by cell IMs & Facebook, so that battle is lost anyway.
Haven't slogged through the thread, so sorry if this is repetitive. My kids were always great around the computers. No food or drinks near them. No crayons all over the screen. They need to learn to use the tools from an early age these days.
cheers,
michael