I didn't really like that scene myself. I thought it was enough to show them triggering the bell once so the son would notice but repeatedly was too much. That kind of thing could cause the kid to have problems later. It may not have been on her mind at the moment but Cinzia actually rigged up that contraption. They could have easily used that as an excuse to go inside. They didn't want it seem they had slept together but that was handled the next morning when she asked the kids if their father was up yet.
Woman, man! That's the way it should be Tarzan.
[Tarzan and his mate]