All children in this study will take donepezil hydrochloride once a day, and will come back to the clinic at 8, 24 and 42 weeks after the study has begun. In between these clinic visits, the clinical staff will telephone the child's parent or caregiver to discuss drug safety observations and possible changes in drug dose. At the end of the study (42 weeks), psychological testing will also be given to determine how well the child is functioning.