When we were designing our website, I found a very good article in Boxer Magazine titled "Are you ready?" I liked it so much I asked if we could use it on our site and was granted the permission to do so. You can read it here:
http://www.savetheboxers.com/ready.html
As a person involved in rescue, here's my insight on your household.
You have an aging female, definately get a male boxer. You could potentially have dominance issues after the puppy grows up and realizes that the aging chow isn't so tough after all. Make sure that the pup does not have 24/7 access to the chows domain. Puppies can be very overwhelming for older dogs. She'll need space to get away from the pup.
You have a cat. Get a puppy or get a rescue dog that you KNOW gets along with cats. Most boxers have a very high prey drive, so adopting an older boxer with the cat would be difficult, yet not unheard of. Both of my rescues want to eat my cats, yet Hammer, who was raised with them likes to play with them. Your cat is already used to your dog, so the transition won't be too hard on him/her.
You children are at great ages to assist in working with your boxer on training issues. Make sure to involve them as much as possible.
In my humble opinion, male boxers are much more laid back than their female counterparts. My female is a live wire and can be quite snippy if she doesn't get her way. The boys just look at me, sigh, and go do something else.
Good luck! And thank you for doing the research before purchasing or adopting. So few do. We've had 3 boxers "owner surrendered" this month to area shelters for the following offenses:
Jumping on people
Jumping on the couch
Digging
Jumping on toddler and knocking them down
The latest is an adorable 8 month old male Harley. Their only crime is that they're boxers! Without proper guidelines and training, boxers will do these things. But that's a whole other topic.