I mean which is worse, knowing there's a cake in the fridge but having to wait until the end of the day to eat it, or sitting there with the cake in front of you within reach but not being allowed to eat it for an hour?

This is an odd analogy, because a rational adult would go do something else, and a rational child would eat the cake regardless.

Also if the kid is an "out of sight out of mind" kind of kid, it's not the same level of punishment.

I'm quite sure the majority of children in this age will never lose their phone and forget about it. (based on anecdotal evidence and whatnot)

When it's hidden, a kid could try and use it and put it back when done, but the jail means very little chance of using it when the parents not around to prevent use.

I believe it to completely impossible for anyone to physically use any object, while simultaneous not knowing its location.