Tobias is $3$ years old. He confidently tells his nursery teacher, 'I am a boy!' However, when he sees his male classmate wearing a fairy dress during play, Tobias insists, 'Now he is a girl!'

His older sister, Clara, who is $7$ years old, laughs and says, 'Don't be silly, he's still a boy even in a dress.'

Discuss cognitive explanations of gender development. Refer to Tobias and Clara in your answer.