The Bullmastiff is a wonderful breed, very calm, and intelligent, not to mention STRONG--- Rollerblades, bullmastiff, and tennis balls DON'T mix (Long story)! My neighbor has TWO of them, and we USUALLY go walking with them, or roller blading with them. They we not raised around kids, but delightfully accept them in my experience, though--- now they know that kids down wipe their mouths very well after a meal, and the first greeting is usually a huge sloppy kiss.
They're just all around wonderful animals, and I think a Bullmastiff, and a Boxer would make a good team.
Good luck. Let me know if I can do anything to help you in your search for one.
Try AmericanKennelClub.com for more information on the breed. Or go to Yahoo, excite or google.com, and type in "BullMastiff", and you should get results.
Take care!