Move your thumb down to meet the middle finger and keep it there until the cramp has faded away. The grown-up grip is excellent for quick writing. It is not as useful for beginners. It is only an advantage, when the hand has developed some discipline. The way we hold a pen today is a novelty.
Some people recommend thick pencils for small children.
I don't think this is a good idea. When I have tried, they take thin barrels, such as ball point refills. How to do it Make sure that the children have the right penhold at the beginning of every class. Of course, some of them will not keep it for long. This need not worry you.