Don't dismiss her feeling as silly or puppy love. She can feel very strongly about a boy at this stage of her emotional and sexual development and if you talk about her feelings for him in a condescending manner she will probably not want to share anything further with you on this topic. Don't dwell on this event or punish her for writing the note. See it as a bridge to communicating your values and feelings about the healthy place of sex and sexuality in a young person's life. These conversations should be part of her ongoing relationship with you. Changing Bodies, Changing Lives, by Ruth Bell and It's Perfectly Normal, by Robie Harris are two good books that can help you out at this time.
Nine-Year-Old Propositions Boyfriend
Toddler and Teenager Expert Advice from Carleton Kendrick, Ed.M., LCSW