It is only normal for a seven year old to be anxious about an enema. Other than
explaining it to her in detail, maybe doing a demonstration and reassuring her that it is harmless, I don't know what to say. You could always do one on yourself to show her how harmless it is....You could also buy a fleet enema, pour out the contents and put water in it and give it to her. It may be less threatening to her.
Also, have her eat an apple a day. Fruits, not citrus fruits, are natural laxitives. Chew it well.