Hi Trevors and welcome to the site!
Personally I think you should be fine with EITHER dog. Both the Boxer and the Lab are GREAT with kids. Both are high energy and will make a good companion.
The good news with your Lab is you should have an easy time getting a quality puppy (since you know the breeder) and you can verify the pup will be taken care of well up to the point you get the pup.
One of the key differences I've noticed between the lab and boxer is the Lab is bred to the bone for playing fetch and ball. Boxers on the other hand..... not so much.
So if you think your kids might like to play a lot of fetch with the dog etc... the Lab might be the way to go (plus your sister is the breeder......)
Good luck with what ever you choose! I love (and have owned) both breeds!
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-Staff
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