Most of the pictures I have seen of omphalitis have involved the abdomen with swelling near the umbilicus. Another possibility might be a leaking air sac. Do you feel anything under the skin like air? A leaking air sac, from trauma, can be helped by inserting a hypodermic needle just under the skin, and pressing out the air. But if the chicks are acting normal and not suffering respiratory distress, the air can re-absorb on it's own eventually without treatment. I am all for not treating unless necessary.