S.,
If your son is old enough to take on the responsibilities of a pet, then I think it would be a great opportunity for you and your family to consider a pet.
For the people who are allergic to cats (and I am one of those people), they are really reacting to the cat's spit or saliva. When cat's clean themselves their saliva dries on the fur and then becomes air born as the animal moves around the house. The allergen is extremely small and sticky and very long lived. Cat dander is extremely persistent within a home. The dried saliva can remain an irritant for 7 years after the animal has been removed from the home. Newer homes that never had a cat living in the house have also tested positive for cat allergens because the molecules are so small, light weight, and stick to nearly every surface. There are a few products on the market that denature (change the chemical structure) of the saliva and possibly make it possible for your family to visit.
Here is a website that sells products to help people with allergies control their trigger points. Some families have had great success using these products, some have not. It just depends on the individual person's sensitivity to a specific allergen. http://www.allergystore.com/
Also, there are many things you can do to make your home "safe-er" for your family when they stay. Keep the guest room cat-free. After washing in a detergent that removes allergens, the guest bedding and pillows should be placed within plastic coverings and put into the guest closet to protect from cat allergens. Hardwood floors are better for allergies than carpeted floors. If you already have hardwood, then all rugs and small area carpets should be laundered in the special detergent and placed in plastic bags. Mattresses and box springs should be encased in dust mite protections while the room is not in use.
About a day before your family arrives to stay with you, you should spray down the walls, floor, and the door of the guest room to denature the cat allergen, then vacuum the room completely to remove the denatured allergen. If you have radiant heat, you might want to consider placing a box fan in the room with a whole house filter taped to the fan. This will filter the majority of the allergens out of the guest room while your family is staying with you. If you have forced air, you should put a very good filter into the vent to prevent the cat allergen from entering the room through the furnace and duct system. (This vent should be closed when the room is not in use) A whole house filter and box fan is also a plus, but may not be needed.
You will also want to spray down the cat itself with a special animal spray that should denature the saliva on the fur. This will need to be done at least twice daily while your guests are staying with you. All of your furniture that will be used, including hard wood furniture, should be sprayed down, carpets vacuumed and sprayed with a special carpet spray. Then everything should be vacuumed again.
The day your guests arrive, remove the mattress encasing, make the bed and put down the rugs and spray all of these down with the denaturing solution just in case the plastic did not protect completely. Take it from experience, it is not fun to be allergic to the pillow you are using at night.
It may seem like a lot of work just for your family to stay. But if it is important for your family to visit and stay in your home, then you need to take your guests allergies into consideration.
Pets are great. They teach children many lessons about compassion, trust, loyalty, and long-term care of a loved one. Our family has a dog that was part of the family before our son was born. Our son is now 8 years old and he is just starting to be responsible enough to take on some of tasks that our dog requires. DS feeds the dog every morning, helps to clean the yard once a day, and plays with our dog each day.
Good luck with your decision.
C. Johnson