What's your budget and how many do you need? Are you also expected to pay for the content or are they old enough to get their own?
If you're already firmly into the Apple world with itunes accounts, iphones etc then the existing 10 inch ipad line is the obvious choice. More cash gets you the later model, faster/better screen etc. The metaphorical handcuffs are very shiny and comfortable to distract you from the cost.
Otherwise, and especially if portability is more important, there are alternatives, especially in the pocketable and increasingly popular 7 inch screen format. The quoted screen size is diagonal in case you're wondering. Bigger screens need more computing power to drive them, and bigger batteries to keep them warm so smaller is generally cheaper too. The app store is where you download stuff from and the alternatives to Apple are the Google play store and Amazon. Highest rated is the Nexus 7, firmly in the Google camp and bristling with nerd chic. New kid on the block, especially if you're already into reading books on the beasties is the Amazon Kindle Fire HD.
Most kids pick up a pretty good idea of what they want from their mates; they don't want 'a tablet' they want a specific one. If you opt for whatever is unfashionable in their circle you could waste a lot of cash.