It's normal for them to be skeptical of new goopy stuff! I didn't feed mine yogurt until they were at least 2 weeks old, and when I did, I made a "mash" with a little water and some of their feed. They loved it from the word go! Like with kids, making it more palatable seems to be the key.
If you see pasty butt, wash it off IMMEDIATELY. Don't think yogurt is going to do the trick. It will help prevent pasty butt, but if you're already seeing crusty vents, take some action! It will kill them.
Several of mine had pasty butt, and I monitored them closely - checked them several times a day for the first week, and immediately wiped it off with a washcloth with warm water. Be VERY careful not to pull at their down feathers on their butts...it can be really painful and lead to bleeding, which can lead to cannibalism. You can even soak their bums in warm water (not too hot of course!). But take action immediately so you don't lose chicks.
Good luck!