We have found that butt paste is a normal thing for very young chicks. In most cases, cleaning it off does the trick but in some cases if not found early enough it could cause them to die. I guess they get poisons not getting it out. Like Wildflower said get a warm cloth/paper towel and get it moistened and get it off of them. May need to soak it in warm water to get it off. You may end up pulling some fluff out as well. Watch them afterwards as they may start to peck each other on the bottom from if they loose fluff. Don't want them to start eating each other.
Check them daily for a few weeks. Shouldn't last long and then they should stop having the paste.
Now in some rare cases, the paste is an indication of a disease but if you are not seeing death throughout I am sure it is simply butt paste and needs to be cleaned off.
Good luck and enjoy them!