I think you're on to something.
First, most people can't even FATHOM living in a house with restricted areas. Not a home office or man cave but an area that is off limits. And not only that but it's accepted, never breached. IMO that's a family will serious issues, probably many unspoken. Major power imbalance. Significant cognitive dissonance. It becomes easier, better to comply, not question. It doesn't happen overnight. Gradual, complex.
Second, I think it's highly probable that the basement was separated. One, accessed from the inside, and one, from the outside. One was RH's private domain.
I suspect his children, curious, may have occasioned to explore it, with shock, confusion and fear.
I think RH took advantage of his family's travels to feed his beast, both collecting victims and perfecting his lair. To account for the time, IMO he lied about the heft of his workload and constructed things around the yard and home, to account for time and purchases.
Think of the pressure be must have exerted to carry off a house of horrors within his family's home.
No one comes out of that unbroken.
JMO