It might have been their breathing, their mental picture or holding themselves still. For a relaxation type of experience, you can use visualization to help a child put themselves in a place that they feel good in. Using that information, you can weave a story with and for a child, using your calm, flowing voice. The presence of someone doing this with a child is powerful to the child. This is a great question, and one that has a many-faceted answer.
All children can benefit from complementary therapies in addition to standard pain therapies.
One of the biggest influences is the family and, specifically, the parent or parent figure. How the family approaches pain, pain relief and has pain beliefs has to be discovered. Most children are very open. The types of families who most often benefit from alternative therapies are ones who have either used them or know about them. We do a lot of teaching, however, in order to give parents and children more tools in their pain care toolbox.