You sound as though you are aware of what the going rate is for the work you are doing, I think its how you approach it thats different with a small/family business.
In a corporate work you'd put together a business case outlining what you do etc, market rates blah blah blah but they don't sound like the sort of people that would work with? Less formal is what I mean.

DH was (and still is but its better now) being really underpaid at his work, which is a small family run business. He his boss that he was applying for jobs elsewhere primarily becuase of his salary, and they increased it by 12% before the next fortnightly pay. No jokes.

I'd try to talk with them about it at the next meeting you organise.