This is a condition know as "pasty butt" - it is fairly common in chicks and is generally pretty easy to deal with. You need to really soak the crusted poo ball with warm water to soften it - you can use a wet cotton ball (get it good and wet and then hold it on the vent area to soak it) or just immerse the little one's bum right into a container of warm water. Let it get good and softened and then gently start working at it to get it to break free. It takes some effort, and they are none too appreciative of the attention, but it must be done. Be gentle and don't pull as their skin is pretty fragile and can be easily torn. Once clean, you need to dry them - you can use a blow dryer on low (be VERY careful not to burn them) or even just towel dry - put them back in the brooder once their fluff has fluffed again as a wet, exposed vent is a prime target for pecking by other chicks. Monitor these chicks and the rest of the group - treat any re-occurrence as needed. Often you see pasty butt when other conditions are stressing the chicks, such as a brooder temp that is too hot, etc., so it can be a good idea to check conditions when you start seeing it crop up.
