“How do I prevent my child pressing a button when they are immediately in front of me”

Sounds like you are either being dishonest on how much you actually watch your kid, or being intentionally obtuse.

edit

even when they're running around like crazy, ready to grab an arm when they move it? And you're never doing anything else in that time?

Your words. While they are running around like crazy - in the kitchen without you being there? Or maybe your washing machine is in some other room?

Shut the door to the room you don’t want them to go into.

Your hands are too full because you’re making lunch? Are you implying you solely focus on that, and nothing else, and in that “one second” they have gone to the washing machine and pressed buttons.

Is the only possible way to stop a child getting to the machine is with your hands? Maybe theres another way … like moving your body to stand between them