There are so many things that the child is miscalculating by using his mischievous mind. He is accounting all the hours he spend separately as well as in the holidays. The weekend are present in the summer holidays as well. 

Then the school last maximum for 6 hours and in that case, even the 4 days that remains will comprise of at least 16 school days. In the similar fashion, the weekends off are not entirely to be excluded but only 6 hours each day to be excluded. 

If you calculate gain, you will be left with so many more days/hours.