Then it became a constant danger about them finding out the truth which we would be brutally punished if that happened because the child would throw a fit due to the abrupt change in their perception/reality and that the tried+true old control mechanism would be lost creating two stupid unnecessary new problems.
Lying lets others control you and prevents you from freely making the most sensible or appropriate decision. It gives a child control over you and handicaps your abillity to transact with them in a way that is productive and authentic.
I'm not gonna say your parent was awful for trying to protect you, I'm simply saying its trades convenience and expediency at the cost of truly dealing with issues and conflicts (assuming you have dealt with the other aspects like child-proofing as best is possible).