。
This kid is quietly sharp, so I really can't say anything careless to her.
Literal
Unobtrusively smart this child [to-には] careless thing [topic-は] very cannot-say.
さりげなく 'unobtrusively / casually / without making a show of it' is the adverbial form of さりげない, a useful adjective for describing subtle, understated behavior. 聡い (さとい) is a literary/elevated adjective for 'wise / sharp / quick-witted,' stronger and more stylistically marked than 賢い. には (composite particle に + は) topicalizes the indirect object. うかつ '(being) careless / unguarded' is a formal na-adjective. とても+can't form (とても...ない) = 'I really cannot.'