I've always got along very well with little kids and I'm sure I won't disappoint you.

disappoint 是动词，由前缀 dis-（表示"否定、相反"）和动词 appoint（任命、约定）组成，字面意思是"使约定落空"，引申为"使失望"。在句中表示"不会让你失望"的意思。