Tumblers are great little birds
I'm sure your son would love them.
There are a LOT of different breeds of tumblers. Since you said you wanted to free fly them, I guess we'll go with the flying ones. Probably the most popular are the West of England Tumblers. There are also Turkish Tumblers, Komorner Tumblers, and more!
One of the best places you'll find tumbler breeds is on Eggbid.com
Oh and also, there are Rollers. People often get them confused with Tumblers. Birmingham Rollers will do backflips in flight, so they're really neat to watch. I used to have many of them, and they're a hardy breed.
You'll need to keep new birds in the loft/cage for a month before free flying. This allows them to get completely settled in and used to the feeding and such. If they're adults, you may also have eggs by then too.
It's not really that hard at all to tell a male from a female. The younger they are, the harder it is to tell. Usually by a few months old, the males will start bow-cooing, there they puff up their neck and turn in circles while cooing. Hens usually don't cause so much commotion. Also in sexually mature birds (5+ months to be on the safe side), you can feel the pelvic bones near the vent. They come down to a "V". If they are very close together or touching, it's a male, and if there's a space, it's a hen.
The parents are very good at their job, so no hand feeding or fostering is necessary. I strongly recommend NOT incubating the eggs since you'll have to hand feed them Kaytee Exact formula from day one, which takes a lot of time. Also, both parents feed the babies crop milk, which basically supplies the young with their immune system during the first week. No formula substitute can match what the parents give the babies health-wise.
The general rule in pigeon keeping is 1 spare foot of space per bird. If you're only looking to get a pair, something like a rabbit hutch will do. But then of course you'd need somewhere to put the babies. Most people have lofts, which is basically like a chicken coop. Here's some pictures of my lofts 'n' cages I've used to give you some ideas:
http://picasaweb.google.com/MaryOfExeter/LoftPictures#
In fact, the one titled "First loft" was originally our chicken coop. But we converted it into a loft pretty easily.
You might find more help looking around in here than what I can tell you. I'm not a builder for sure
Lot's of loft plans, pictures, and info:
http://www.pigeons.biz/forums/f38/
For food, anything from wild bird seed to chicken pellets will work. Pigeons love a variety of seeds, so something more bird seed-like is probably better than say...chicken scratch. Some feed stores carry pre-mixed pigeon food made by companies like Brown's. Pellets can make birds fat, so it's best to mix it with seed. Pigeons can take a while to get used to eating pellets, so don't get discouraged if you do choose to feed them some. On another note, pigeons love peanuts, so if you ever want to give them a treat, just throw in some raw spanish peanuts. You'll also need to provide them with grit. Chicken grit is fine, preferably with some cushed oyster shell in it. Grit is almost as important as the food since it helps with digestion and is a good source for calcium and other minerals they may not get from their food.
And finally, YES they are winter hardy
As long as you have an area they can get in with at least three solid sides so the cold drafts don't get to them, they'll be fine. Pigeons can get acclamated to just about any climate. The biggest thing that can hurt a pigeon in the cold is the wind, since it doesn't take much to cool a bird's body down really quick.
If I missed anything or if you have any more questions, I'll be more than happy to help!
Talking about pigeons is fun to me.
