This is an honest mistake with some names that are gender neutral, or if a child has a gendered name but isn't what you expected. I would need more information because I don't have the whole picture, and your question is a little unclear. Was this a transgender child? Did you hurt the child's feelings? Was the child upset? Did you use the wrong pronouns? Did you call for the child and say I'm here for "XYZ, is she here?" If so, sincerely apologize for the error and move on. If not, and you made an assumption, double-check everything, check with your supervisor (if they already know the child), and ask the teacher/parent (I'm not sure if you are in a school or a healthcare setting) and say you are looking for "XYZ" child.