-looking for first puppy - You might be okay with a Boxer here if you are truly commited to providing it with plenty of training, socialization and (mental and physical) exercise.
-medium sized dog - Check, but don't underestimate a Boxer based upon it's size.
-limited/low shedding (important) - Boxers shed. Heavily. Dogs that do not shed as much have coats that grow continually and thus require regular brushing, combing and trims. Think: Poodles, Portuguese Water Dogs, Soft Coated Wheaten Terriers, etc.
(Sorry, BoxMeIn21, but even when we fed raw and I curried and stripping knifed them every week, and thoroughly bathed and groomed them every month, my ex's Boxer and Boxer mix STILL shed WAY more than my Poodles who are truly on the low end of the shedding spectrum.)
-okay with my 6 year old daughter - This would again depend upon your commitment level to the puppy and also on your daughter's behavior around the puppy. I would actually suggest an older, trained dog so you can skip having to deal with a puppy who will undoubtedly be more energetic, mouthier, pushier and bouncier than you can imagine.
-we live in a house with a large yard - That's good, but it won't be nearly enough to keep ANY dog happy.
Boxers are VERY prone to some serious health issues. Getting a pup from a breeder who shows, health tests and guarantees their stock IS VERY important. Whatever you do, research the breed well before you jump in to getting one.
http://www.boxerworld.com and
http://www.americanboxerclub.org both have some excellent information for you to consider.