100$ for a Kindle Fire is a decent buy. The market is plagued with cheap tablets that may look good hardware wise but have HORRIBLE battery life. I'd rather have a tablet like the Kindle with no camera, bluetooth or SD card expansion than a tab with a 3 hour battery life. On JB ROMs I could get around 8 hours of continual use and 10+ with CM7 ROMs.
Another good thing about the Kindle Fire, like stated earlier, is the strong development base that exists for it. For a 10 year old child the Kindle is perfect: No camera, no SD slot to become clogged with debris and can easily play videos and music. My 3 year old niece loves to play the Kindle and can navigate it with ease.