how to restrict my grandson's ability to purchase games on family plan

Purchases made in Family sharing would appear in the account of whoever initiates the purchase., not the family organizer's account even if they are charged to the Family Organizer.

You can set up ask to buy for children under 18 to require authorization for purchases.

click here ➜ Approve what kids buy with Ask to Buy - Apple Support

My understanding is purchases will only appear in the purchase history of the person making the purchase, but the Family Sharing Organizer will receive a receipt if the Organizer's payment method was actually charged.

This support article outlines how to actually restrict purchases Approve what kids buy with Ask to Buy - Apple Support