How do I tune a trumpet?

The main tuning slide, the large U-shaped one nearest the mouthpiece end, sets the overall pitch: out to flatten, in to sharpen. Warm the horn up first, because a cold trumpet plays flat and you will chase your tail adjusting an instrument that just needed a minute of playing.

The first and third valve slides exist to correct individual notes that are naturally out of tune on every trumpet. Working those is technique, taught by the teacher in due course. A parent never needs to touch them.

The practical part, and the one that saves repairs: if any slide will not move, the answer is grease, never force. Slides that get worked and greased now and then slide for a lifetime. Slides that sit untouched slowly seize as the metals fuse, and a truly stuck one is a workshop job involving heat. So make the tuning slide part of the routine: pull it, wipe it, a touch of slide grease, back in, once a month. Or hand it to us across the counter. It takes a couple of minutes.