The child showed a big Kool-Aid smile after getting a gift.
Kool-Aid smile
idiomA large, beaming smile.
Origin
disputed originFrom the wide smile of the character used to advertise the drink.
She walked into the room with a wide Kool-Aid smile, clearly proud of her recent promotion.
Despite the long and difficult day, he greeted his family with a bright Kool-Aid smile that masked his exhaustion.