Many (but not all) people with Down syndrome develop bunions. A bunion is a painful bump that forms at the base of the big toe. Surgery is the definitive treatment for bunions but there are several things you can do to try to delay or avoid surgery, as the article linked here describes. 

Like with many other painful conditions, people with Down syndrome often do not complain of pain with bunions. If a person with Down syndrome is not complaining of pain or demonstrating pain, then the main indicator that a bunion needs to be addressed could be a change in a person's ability to walk or participate in activities. Treatment can improve function as well as pain.