Give your child a handful of clothes pegs and something with a firm edge — a shoebox works well.
Show how to squeeze a peg open with thumb and finger and clip it on. Do one, then pass the box over.
Count them on together as they go. Taking them off is just as good a workout as putting them on, so let that be a second round rather than tidying up.
A clothes peg needs real, sustained force from exactly the thumb-and-finger grip that later holds a pencil. Few household objects load that grip so directly, and the peg gives clear feedback when the squeeze is not enough.