The couple was supposedly trying to hide the kid, but the real state agents and thus people interested in buying the house have been informed about him. Even the room is labeled as "the children's room".
This one at least is possible.
Saitama is kind of bedroom neighbourhood to Tokyo. It’s possible that the residents typically went out from morning and only return at night to sleep, so interaction between neighbours would be minimal.
They don’t need to hide about having a child from the real estate agents, nor hiding the hidden room from prospective buyers in the future. The family wouldn’t be interacting with them that much. It’s kind of like the windows trick, if they don’t act like there’s something to hide, people typically won’t feel suspicious.
The important part is, keeping the neighbours from noticing, which wouldn’t be so difficult. If they never saw the kid, they will just assume this couple got no child. Not like they will check their neighbour's floor plan like a creep.