I don't know if there is a "best" baseball mitt for a six year old, but there are some guidelines you should follow when looking to purchase one. One of the most important aspects is that it needs to fit correctly. Gloves that are too big are too hard for younger kids to control.
Another important thing to look for is that the glove has a decent sized pocket. The pocket should be at least the size of a baseball, so the ball stays put when it is caught. It is very frustrating for young players to play catch with a mitt that has a small pocket because they always drop the ball.
I would also recommend not spending a bunch of money on a baseball glove for a six year old. You can definitely find a glove for under $35, but most will be way under that. Kids that age will grow fast and the quality of the leather (important aspect of the cost) is not that important.