To start, just buy them reliable little bikes (eg Yamaha PW50, Honda CRF50) that they can have fun on. Almost anything will do. Buy something Japanese. As Joe said, you can get new ones for less than $2000 each. Auto clutches would probably be good. My kids started with clutches, but they were about 9 and 10 at the time. They should be able to comfortably touch the ground. The bikes need to start easily and run trouble free, or the kids, and you will get frustrated. When they have learned to ride a little bit, then buy them something of higher performance. Don't worry, they will grow out of everything really quickly(unfortunately). Make sure they have sturdy boots, gloves,goggles, long sleeve shirts and decent helmets. Hard to believe, but one of them might not even like riding (can't imagine that happening to both).My 14 year old is on his third bike and finally this year has conceded that a bike is more fun than a quad. I resisted buying him a quad because they are dangerous. My 15yr old has never wanted a quad, he "got it" right away. Lots of kids will be frightened if they can't touch the ground or if the bike is really powerful(and some of those little race bikes really are). Your budget is perfect for a couple of new starter bikes. They hold their value pretty good, so you won't lose much on them. Start saving for bigger bikes. It's a trap. You'll love it!