Summer is an important time for learning in children. Studies show that children lose up to 3 months of the equivalent of what they learned in school over the Summer. This often puts schools at a disadvantage, because they have to take the first 3 months of school to catch up. https://en.wikipedia.org/wiki/Summer_learning_loss
This is most apparent in math and reading comprehension. In order to help young people stay abreast of their learning, Summer is a great time to bone up on areas that students have struggled with. Consider using free learning sites such as Kahn Academy [khanacademy.org] to help young people practice essential math facts.
Summer is also a great time to get kids interested in learning. Consider bringing them to the library and picking out some good reads. During my 6th grade summer I found the joy of reading as I fell in love with C.S Lewis Narnia series. Children can learn programming, join a chess club, and do outdoor activities.
Do not forget to have a lot of fun and instill this fun with learning. When going to the movies ask the child to describe their favorite parts of the movie. Encourage them to go outside and take pictures of natures...etc.
The main thing in the summer is to keep children active mentally.