Hi, Terry,
You might try the free download, Handbrake, to make an Mp4 copy of the DVD home movie. You can get Handbrake here:
https://handbrake.fr/
It is a very reliable app that is used by many on this forum.
First, insert into your Mac's optical drive the DVD disc that you want to copy. The disc's icon will appear on your desktop. Then open Handbrake and do a File/Open Source. In the resulting screen, Navigate to the DVD disc's icon on your desktop and choose it as the source. Then do File/Start Encoding. Wait a few minutes for the conversion to complete. Handbrake will copy the DVD and an Mp4 movie will appear on your desktop. Then save and import the converted movie clip into iMovie for editing.
Another way to copy the DVD into a movie file is to click on the DVD icon on your desktop to open its library contents. You will see a VIDEO_TS folder and an AUDIO_TS folder. Ignore the AUDIO_TS folder. Open the VIDEO_TS folder. You will see several files inside. Copy (copy, not move) to your desktop all of the files that have a .VOB extension. Ignore the others. Then use Handbrake to convert the .VOB files, that contain the DVD video and audio, to Mp4 files that you can import in iMovie and edit.
I would go with method No. 1, because it is easier. If you are not satisfied with the quality of the result, then try method No. 2. The quality should be fine, though. However, that would be your call.
-- Rich