I guess it all depends on the kid, and the other sections of the child kid relationship. Some kids can handle finding out they were being lied to, some can't; some parents can manage to lie to their kids in a way that doesn't set the kid up for a major fall, some can't. If my mom ever tried to lay that kind of absolute on me there would have been open rebellion, I've never done well with ultimatums.
Jeepers, it's not an ultimatum, it's fun. Is there no such thing as a wink and a smile in your world?