Give your child paper you are happy to lose and a bowl to fill.
Show one tear: two hands, thumbs close together, pulling in opposite directions. Then let them go at their own pace.
When the bowl is full, tip it over their lap or a cushion for the crunch. Scrunching each piece into a ball afterwards is a second, harder exercise using the same paper.
Tearing requires the two hands to work against each other with different forces — bilateral coordination in its clearest form. Scrunching then loads the whole hand arch, which supports the grip strength handwriting depends on.