Agree it is risky, but very likely that you can indeed get away with it , especially if it is just for one night (not that you should proceed with this plan). Japan hotels are usually a stickler for occupancy counts unlike in the US. There are probably occupancy regulations/laws surrounding this.

Depends how obvious your kids are when they enter or how much attention they attract. You enter from the ground floor to the ?32 floor lobby. You do have to pass the concierge and check in desk but depending on time of day it may be quite busy. For a 200 room hotel, they can't pay attention to all the guests or what they look like. Who is to say they are not a day guest of a guest? I had friends who I escorted up to my room to visit and no one batted an eye.

I guess the most obvious part would be when they are towing luggage. Definitely give all the bags to the bell desk to send to your room, if you decide to proceed with this. (Again, not that you should)