I suggest that you try several heights and build whatever is most comfortable for you for working above and below the benchwork. Visit some layouts in your area and ask the owner how he or she selected the height and if they are OK with it.
The trend with many model railroads today is to build them higher rather than lower. Higher layouts are easier to work under for wiring, switch machine installation, etc. You can store more stuff under a high layout. Also, people like the eye level view.
A low level layout built for the kids will soon be a pain in the back or knees because kids grow fast. It will be too low even for them in a few years. The older I get the more I appreciate a high layout. NH Joe