Think through the consequences.
Whenever you make a ruling on a subject like this, remember that you are setting a precedent not just for the party, but for your whole world. If conjuring wizards are common in your world, then anyone wanting to lock things up are going to be extremely careful not to let anyone see their keys. If such wizards are rare, then they won't.
If you are paying attention as DM, you should respect a law that I call the "Conservation of Difficulty". If Knock spells and skilled lockpickers are common in the world, then so too will be locks with higher-level protections. If charm spells such as Suggestion are easily available, so too will be protections against them (and probably a whole set of laws, courts and prosecutors to deal with such "charmers".) Whenever a player uses a spell or ability "by the book", it should not change the general level of the challenge - first level spells should not have third-level effects. Only if the player is truly creative, going beyond the obvious application of the spell or ability, should the challenge be bypassed. Otherwise you should demand more from the player - a higher level spell or effect, a higher roll on the dice, or some additional actions such as getting a close look at the key.
If you don't respect this law, your players will surely notice. If this room is supposed to be highly protected and they get in using such a simple technique, they will ask themselves, "Why wasn't this room plundered before?" And most players will feel a little bit robbed. Unless, of course, your answer is, "Well, it would have been hard for anyone else - you are the only conjuration wizard within 100 leagues", or, "Well, it was supposed to be easy - it's just a mop closet." Or maybe, "You won't realize it until later, but the mayor wanted you to get in, and since he knew you were a conjuration wizard he simply made a point of letting you see the key. He showed you the bait, and you took it!"