Add a few drops of acv to their drinking water and run the water warm and hold the chicks bottom under . Gently loosen any poop with your fingers. Boil up a few eggs , cut them up or mash them with a little plain Greek yoghurt. Check that your brooder is not too hot, pasty butt is often caused by excessive heat and / or dehydration.
I sit with mine , the lower to the ground the better, sprinkle food treats around and let them come to you. Chickens prefer to be lifted from underneath as a posed to being grasped from above, ( predatory thing ). Here's a link with correct temps and other things to look out for.
http://www.the-chicken-chick.com/2012/03/baby-chick-basics-what-you-need-to-know.html