1. How easy is it to edit the videos? What's the best software? (loaded question, I know!)
"Easy" is a relative term. I was REALLY surprised at how easy Apple's iMovieHD was to use when I first started several years ago. FinalCut, on the other hand, has quite the learning curve. I suspect the same can be said for MovieMaker and the purchased Windows apps like Sony Vegas or Adobe Premiere and others.

But I think you missed a step - that is, getting the video into the video editor. If you are married to getting a hard drive (or flash memory) camcorder, copying the files over a USB connection is pretty easy. The files are "in the computer". What happens after that is where it can get exciting. Some video file types are easier to get into an editor than others. Without knowing which camcorder(s) and video editor you are considering, it is difficult to know whether getting the video into the editor will be easy, or not. Sorry - I hate answers like this, too.

The BEST software? Yes, loaded question - but iMovieHD, FinalCut (Macintosh) and Sony Vegas, Adobe Premier and a couple of others (Windows) seem to always float to the top.

2. For the memory card cameras, can I just plug it into my TV and watch the videos from the TV's memory card?
My Panasonic HDTV has an SD card reader - but it likes JPGs only - no other still formats and certainly no video formats. Which TV do you have? The file formats that card reader in your TV can use will be in the TV's manual.

3. Do I really need HD to film my kids growing up? I'm an HD buff, but I just don't know how well these new camcorders are and if it's worth the expense.
That is purely your decision. I wish high definition was around when my son was a kid. I guess the question *could* be, "Do you really need a camcorder?" I don't think anyone really NEEDS any of this stuff - we WANT it, sure, but "need"? It is all in your priorities and budget.