How do I tune a clarinet?

A clarinet is tuned by adjusting how far the barrel is pulled out from the upper joint. Out to flatten, in to sharpen. Small movements: an eighth of an inch is a lot.

Two things matter more than the adjustment itself. A cold clarinet plays flat, so let your child warm it up (literally, warm air through the instrument) before judging anything. And embouchure moves pitch more than the barrel does: a student biting too hard plays sharp, and no amount of pulling out will fix what is really a technique matter for the teacher.

Which is the larger point about tuning any wind instrument: intonation is a skill the teacher teaches, not a setting a parent adjusts. Get the barrel to a sensible starting place with a tuner app, keep the corks greased so the joints move without a fight, and let the teacher handle the rest. If the barrel will not move at all, the cork is dry. Grease it rather than forcing it, or bring it by and we will sort it while you wait.