Not to be a killjoy, but I think you maybe ought to think about whether you even want a heater.
I have a bulkhead mounted diesel heater on my C30, and I've used it exactly twice in the past 3 winters.
Our kids are about the same age, and I don't know about your family's attitudes, but in general, if it's cold enough to require a heater, it's too cold for sailing.
When we cruise in the winter, it's generally weekenders to "civilization" at a marina with shore power. We just use an electric space heater then.
So, I wonder what use you will really get out of it...
I'm glad I have heat on my boat, but am not so sure that I would pay to add it.
Of course, you ARE farther north than Edmonds....