Add cocktail sausages to your slow cooker.
In a medium bowl, whisk together BBQ sauce and grape jelly until smooth. Add red pepper flakes if using.
Pour sauce mixture over sausages and stir gently to coat everything evenly.
Cover and cook on LOW for 3 hours, or HIGH for 1.5 hours, stirring once or twice during cooking.
Once sausages are heated through and sauce is thick and glossy, switch to WARM setting.
Serve with toothpicks and keep on warm for easy snacking throughout your party. The sauce gets even better as it sits!