I have the HG21. If you're concerned about size you'll have to transcode them into another format. Keep in mind that while you may be using iMovie now you may graduate to something more robust in the future like Apple Final Cut or Adobe Premiere.
Both of those have the ability to read the raw AVCHD files and pull segments out of them without having to fully import the entire file. This is important because of the size of the AVCHD files to begin with. Dumping a couple of gigabytes fully into FCP or Premiere when you only needed a minute of video can be burdensome on the editing process and the storage needed for the project.
Premiere and FCP will read out of a saved AVCHD directory natively. It treats it like the drive in the camera. As long as the directory and file structure is intact it will allow you to access the files as described above.
If you don't think you'll ever move on to a more significant program than iMovie I'd say to transcode to another format significant enough for the best output you intend to create. That's what you will need to decide. Will you be rendering for an HD TV, a SD monitor or the web? If it's the web will that original clip ever be used for anything better? You'll need to target the best format you may eventually use. Never go lower because you can never get the lost resolution back.
If you must transcode out of AVCHD then I'd go with the Apple Intermediate Format. If that's still too large for you then a higher resolution H.264 format will have to do.
As for future-proofing your storage, that's a little more difficult because we don't have crystal balls. AVCHD is newer for the consumer market. the HV10/20/30 used MPEG-2. Experts have differing opinions as to how AVCHD will hold up as a format. It's cumbersome to edit and today's computers can have a very tough time keeping up with the bandwidth. Your HG10 maxes out at 15Mbps where the newer models are now 24 at a maximum. 15 is much easier to get along with the average computer.
In the end, you could go with H.264 but who knows what may come down the road. You can always stay on top of the current preferences as time passes and convert again as needed. Drives just keep getting bigger so storage won't be an issue.
-a semi tech-savvy father whose wife will kill him if he loses the camera footage
Probably most importantly you aren't addressing back-up. If you have only one copy of something you don't have a back-up. If you're worried about being in the dog house, what happens if that WD MyBook fails? Hard drives fail frequently. You're going to be eating Alpo. You're going to have to come up with a back-up strategy. That drive data is going to have to be backed up on another drive as well. I have a 1TB drive I use for project management. I will do my editing there and my original copies stay there. I have a NetGear ReadyNAS network storage drive that retains all my back-ups. It has 2 1-TB drives in it. There is 1TB of storage and 1 TB mirrored back-up. That way I have my original files plus two backups of everything. I've lost too many files in the past to let it happen again.
A co-worker of mine had a hard drive fail about six months ago and she lost all of her digital photos. She got the drive replaced and bought an external drive. Her replaced drive failed a few weeks ago. She still ended up with nothing because she never did anything with the external drive. She claims she has learned her lesson now. We'll see.
If these files are important to you then you really need to come up with a back-up strategy along with your decision about what format to save in.