Not all 5-year-olds are equally careful or easy study's. I don't know if this would be too 'kid friendly', but you might check out the 'One Laptop Per Child.
http://laptop.org/en/ I think. These are made to take abuse. Run on minimal power, are networked, have no hard drive and use Linux. Nicolas Negroponte conceived these to provide to kids in 3rd world countries for (originally) $100. The price is up a bit now, and the last I heard, if someone in the 1st world buys one, another is provided to a kid in Africa, India, or somewhere else I can't remember.
I think the ASUS eeepc would also be appropriate for small fingers. I saw one at Target this week for about $300. I have one and find it great for travel and light use. The keyboard is a bit cramped for my big fingers to type 'War and Peace'.
You may also look into used (reconditioned) computers at a local retailer. I think I might not give a 5 Y.O. a laptop, because desktops are more easily (cheaply) repaired and generally more durable.
Jim Ricks