I already have a few WDTV's around the house along with xbox's etc and stream everything on my PC to them , it should be an easy conversion!
This is where you need to temper your expectations. If your idea is to store digital files on your pc and send them around the house thats no problem. But if you think you are going to be able to use your PC to access to access MTV.com, and then send The Real World to your WDTV, you are going to be disappointed.
There is a service called PlayOn that attempts to do that, but I've heard a lot of mixed things. And it costs money. So to get the best experience you really want your devices to support the services you plan on using (the 360 is a good one).
* Kid channels (Nickelodeon perhaps, etc) It's easier to put a channel on for them than to pick a movie or show every 20 minutes plus
Netflix has a ton of kids programming from a variety of sources. You probably also get PBS and Qubo over the air. Your local library probably has a lot of kids dvds you could rent.
* Football / Baseball games
* ESPN (I usually watch Mike & Mike in the mornings, along with Baseball tonight, NFL Primetime, etc)
Sports is going to be the last bastion of pay tv. You will be able to watch the Super Bowl OTA on NBC (and I saw a quick blurb that they will stream it online too). But other than a few games on Fox you will need to subscribe to MLB.tv, and thats only going to work if the team you want to watch isn't in your geographic area (and isn't on a national channel like ESPN). So for example I could subscribe to MLB.tv here in Pittsburgh, but I won't be able to watch any Pirate games because local channel Root Sports has spent a ton of money to offer them exclusively.
ESPN programming can only be seen online with a pay tv subscription to a package that includes ESPN. They have their free ESPN3 online channel that shows foreign sports and some college stuff, but theres no way to get Sportscenter and the like.
* Normal broadcasting tv, if possible (we all love to just turn the tv on in the background, dont we? Or just flip channels) - This isnt a huge deal if we cant
Antenna. Well, antenna if you live relatively close to the broadcast towers. I get all my locals via antenna (except my ABC affilate, which comes in poorly). My newer Samsung tv actually has a really nice guide and I think I can record to a thumb drive (but I haven't really tried that yet).
* Her shows: Most reality shows ( Housewives, Real World, WWE Divas etc etc etc) - I can get most of them, but some I can just never seem to find
Some of this stuff is probably free on their websites after it airs. A lot of it will be behind an authentication wall like ESPN, where you have to log in with your cable tv company. If there are a few you really like you could buy a pass from iTunes, Xbox Video, Amazon Prime, etc.
Bottom line is that there really is no way to get a lot of premium pay tv content for free or even low cost (especially sports). You can buy episodes on services, but those cost a few bucks each. Netflix, Hulu, and Amazon all have a lot of tv shows, but (other than Hulu) they don't tend to add them shortly after airing.
Dish's new Sling TV product might interest you. Its going to be about $20 a month and will include streaming access to a few tv channels (ESPN being the big one).
But another thing you need to consider is what will happen to your internet bill when you cancel cable tv. Comcast will raise it at least $20, taking away the "multi-service" discount. You might save money by keeping "basic" cable (local channels only) and subscribing to the internet than internet alone.