On my last trip to the Netherlands (2018), someone I encountered en route recommended the middle band (Dordrecht - Nijmegen sort of thing) to me. I've done the coast a few times, and it's lovely, but I might take that advice and explore that way a bit more next time.
I did make a bit of a foray inland for a few days, and did really enjoy pootling around in the forests. A lot of the mountain bike trails turn out to be very doable on a laden tourer and broke up the monotony of exquisite cycling infrastructure a bit
I did some border crossings one day that took in some of the trails just North of Arendonk, then West along the canal to Turnhout before heading back up through the Baarle-Nassau enclaves. Would something like that provide enough en route interest for the kids? ("School age" covers quite a spread!)
I enjoyed riding through the Chaamse Bossen forest too.
I mostly used
https://www.natuurkampeerterreinen.nl/en/ with a Warmshowers host in one of the cities I went through.