Good Morning H..
I worked in cat rescue for about 3 years and am going to add my voice to the mix.
I believe that 12 years old is a good age for a first pet. It is a good time for the child to learn about the value of 4-legged life and additional responsibilities. The decision, of course, is yours. You may find yourself doing some of the upkeep of the animal in question. There is a "Cats for Dummies" book out that has a ton of information (much more than any of us could post here). I also have "The Ultimate Cat Book" that I picked up at Half Price that has pictures of a bunch of breeds in it. It is good because it shows you the length of the hair, tells you some of the dominent personality traits, and gives an idea of the grooming upkeep of the cat. They also suggest going to a cat show to look at various breeds to help you see them in action and help make a decision.
I am very opposed to de-clawing. Working in rescue, there were many sweet cats who became biters or developed other undesirable traits following that surgery. It is the cutting of the tendon that extends the claws or the actual removal of the claw itself. I see it as mutilation. Most kittens are taught how to use their claws in play appropriately by their human companions. I have cardboard scratchers that all 3 of my furbabies love. It also looks and feels incredibly different than the carpet so there is no confusion there.
A Manx according to this book is a tailless cat (for the most part. There will be the occasional tailed Manx. The taillessness is caused by the fact that the cats were confined to the Island of Man with limited breeding partners. Coat-double, well padded, Grooming-easy; regular brushing, Temperment-calm, intelligent, active, loyal; likes to be with its owner.
I also recommend keeping the cat indoors. Changes in environment can cause excessive shedding and cause the skin to release more dander. Most allergies are not to the actual hair of the cat, but to the dander that all cats, dogs, and humans shed. Even the Sphinx and the Rex breeds still shed dander. Regular or daily brushing helps to regulate the shedding of hair and dander. It would also help your dogs if they were brushed regularly. The most important thing is to make it routine, part of the day. Works better for the cat.
Ultimately, the question you are going to need to ask yourself tho, is do YOU want a cat?
~C.~