Bringing up counselling can feel like a bigger conversation than it needs to be. Most children and young people take their cue from how calmly the subject is introduced, rather than the words themselves.
A simple, low-key framing tends to work best: describing it as "someone to talk to who isn't family or a teacher" rather than anything clinical. Avoid over-explaining or promising a particular outcome — young people often respond well to knowing it's their space, and that they're allowed to find it strange, boring, or unhelpful at first.
It's also worth saying plainly that they won't be in trouble for anything they say, and that you won't be asking for a play-by-play afterwards. That reassurance alone often lowers the pressure enough for a first session to go ahead.