I’ve discussed before how helpful it is to have kids hold a pom pom or toy in their palm while writing. It gets then to stabilize the pinky side of their hand and frees up their index, middle finger, and thumb for controlling the pencil.
One problem is that they lose the object they need to hold onto, so my solution was pencil charms. You attach an object to the pencil so that it is always there. These work great for many kids.
I just found these straw caps on Amazon that could work the same as pencil charms. They aren’t as tight on the pencil, but they do work well. They are not stretchy though, so they will only fit on a regular size pencil. Try them out and see what you think.