Is a stage box with an iron grid safe using this way?
There absolutely is no issue in using a correctly rated momentary switch to turn on and off a lamp directly. After all, our houses are full of wall switches, foot switches, and even not so safe kinda sloppy switches from non controlled markets.

But there's a catch: there is no issue if you can ensure that the switch will operate in a proper environment at all times. If you can understand what a proper environment is, and enforce it, then go ahead with whatever design comes to your mind.

Using a low voltage switching circuit is certainly a hassle, because you need a transformer, a PCB, whatever: it's a mess.

But consider this: you can buy an off the shelf relay, connect the mains voltage to it, put everything in sealed box like what they use to make electrical connections in gardens, and just run around your low voltage control wires.

With low voltage control you can do a number of things that are more difficult and costly to do with the mains directly: you can add crappy foot switches everywhere, and enjoy the light going on and off when your drunken guitarist drops a pint on it while smiling to that girl that was "definitely looking at him". You can add alwasy on, always off switches at no cost. You can parallel foot switches so that you can turn the lights on and off from various points in the stage. You can add a microswitch to your drummer pedal and have cool, perfectly timed, blinking lights. You can even program an arduino or whatever to produce super cool effects in the future, if you want.

Be safe on stage, if you have to ask if it is safe assume it is not safe.