Sweet potato is not really a potato but a root vegetable. Its better for them, and us, without cooking. But if you cook your's just make sure its not too hot for your little angels...especially if you use the microwave; it can be deceptive with really hot spots...
If Momma were in the brooder with them, she would make a happy churrup like sound and let them know it is okay to eat it. With out the momma, you will have to just offer bits to them to see what they like and dislike - Yogurt is a good one with some crumbles in it. A little bit goes a long way. Crumbles make everything more inviting....I tease my daughter about sprinkling crumbles on her veggies! The chicks know the crumbles and that helps them to experience other things. Yours are kind of little still but they will double, triple and quadruple in size as the weeks go on and may enjoy treats. It really isn't necessary to give them treats now. They like red things best it seems. Watermelon is good treat. Small meal worms!
A little cooked and mashed sweet potato is fine. But only a little. Chicks that young need to eat mostly chick starter. Sweet potato has a lot of carotenes, a good bit of sugar, but not a lot of anything else.
I introduce my ducklings to as many different types of food as possible at a young age. They ALWAYS have access to feed, but I tear up dandelions, give them clover, peas, beans, celery trimmings, grape leaves, etc... and toss it in their water dish every day. This leads to ducks that as adults will happily devour your kitchen scraps and weeds. I have a 10 week old drake I just got who has no clue what to do with veggies. He looks genuinely puzzled at the ducks I raised from babies as they scarf down their morning "salad".
I learned this, not only from raising my own kids, but from raising tropical birds. If all they ever get young is dry seed, they will not eat the nutritious greens and fruits they SHOULD ALSO be eating once they are adults.
Half of the benefit of owning poultry is their "garbage disposal" skills. You gotta learn em young