Shrewder
The definition of shrewder is having or showing sharper powers of judgement; astute.
In this case, this refers to the fact that Atticus knows that Scout is tired even when she tries to deny it. Also, that Atticus lets Scout stay up because he knows that she will soon fall asleep of her own accord. Here Atticus judges it will be easier to get Scout to bed if he allows her to stay up a little later, and then realises her tiredness herself.
In this case, this refers to the fact that Atticus knows that Scout is tired even when she tries to deny it. Also, that Atticus lets Scout stay up because he knows that she will soon fall asleep of her own accord. Here Atticus judges it will be easier to get Scout to bed if he allows her to stay up a little later, and then realises her tiredness herself.