Your suggestion for mommy to take an enema while the child observes is an excellent one, and something I have suggested to parents in the past. In fact, the best time for this is when the child is very young and doesn't need an enema at the time, but it provides the opportunity for the parent to explain how the enema can help us when we are not feeling well.
This way, should the time come in the future when the child would benefit from an enema, he or she won't have a fear of the procedure.