A white-knuckled grip and a hand that aches after two lines is almost never a child being difficult. It is usually a hand that is not strong enough yet, compensating.
When the small muscles of the hand cannot hold the pencil steady, the child recruits everything else — fingers, wrist, shoulder. The grip gets tighter because it is the only control available. Telling them to relax removes the control and gives nothing back.
The things that help are not writing. Play dough. Pegs. Tearing paper into strips. Threading. Squeezing a sponge in the bath. Cutting with scissors is one of the best, because it works exactly the muscles a pencil needs.
Ten minutes of this beats an hour of extra writing, and the child does not experience it as work.
A hand that hurts is telling you it is at its limit. Three good lines then stop, daily, builds more handwriting than a page that ends in tears — and it keeps the child willing to pick up the pencil tomorrow.
A mature grip commonly appears somewhere between five and seven, and the range is wide. If a child is still using a fist grip well into school, mention it to the teacher.
They help some children and are ignored by others. They are cheap to try, and they are not a substitute for building hand strength.
The same small hand muscles do both jobs. It is also far more appealing to a child who has decided writing is the enemy.
Part of our guide to Handwriting & early skills.