Normalize the name difference early. Toddlers usually learn to say their first names around three years of age. That's also the best time to teach your child his last name and yours. Just be matter-of-fact and brief: "Your name is Sally Smith. Mommy's name is Laura Kelly." Then answer questions as they are asked. Young kids don't need lengthy explanations.
Explain your motive. If you choose to change your last name, explain your decision in simple terms your kids can understand. They can paraphrase your explanation to any who might ask why their name differs from yours. A divorced friend told her kids that she was taking back her maiden name because it was her birth name and had special meaning for her. No further explanation was needed: her kids were satisfied.
Visit the school. Whenever you enroll your child in a new school you must show legal documentation, including your child's birth certificate and guardianship papers. Make sure the documentation accurately reflects the name situation. Also, if your child has any special medical needs (i.e., peanut allergies, asthma) or requires prescribed medication, alert the school nurse and provide necessary emergency contact information for her as well.
Talk to the teacher. Teachers will call students by the legal name listed on their roster. If you want your child to be called by a different name, alert the teacher before your child enters her class. Otherwise you may have a distraught kid on your hands, wondering why the teacher is calling her by the "wrong name."