When I first heard the term "summer slide" I thought of equipment on a playground. But as I'm sure you're aware, there's another meaning entirely. This slide refers to summer learning loss.
There's lots of research about it. Children tend to lose reading (and math) skills over the summer when they're not used.
There are many activities that enhance reading and will slow or stop that slide — talking, singing, reading aloud, keeping a journal or photo album of summer activities, and lots more.