Many children will complain of a pain in the hand when writing. It is important to have a conversation with the child about the nature and location of the pain. Some children will describe the normal discomfort experienced when writing as pain.
We all shift the pencil around in the hand whilst writing - this is done subconsciously. If you cannot do this due to poor fine motor skills your grip will remain static putting pressure on points of the hand and overworking the same muscles. This causes discomfort and fatigue - variety of positions relieve this.
Try:
Thumb loop splint
Thumb post splinting service