Cook sweet potatoes in boiling water to cover 30 minutes or until tender. Drain and cool. Peel sweet potatoes, and mash.
Combine sugar and next 3 ingredients; stir sugar mixture and egg into mashed sweet potatoes. Shape mixture around marshmallows, and roll in crushed cereal; place in a single layer on a baking sheet. Cover and freeze 2 hours.
Pour oil into a large skillet to depth of 1/2 inch; heat to 350°.
Fry croquettes until golden brown, turning once. Serve immediately.