Yes, it is possible. In some rural areas or in families with more liberal attitudes towards nudity in nature, a kid might have spent the whole summer naked. For example, in some small villages near the beach, kids may run around naked while playing close to home, as long as it's in a private and safe environment.
One reason could be the hot weather. If it was extremely hot during the summer, being naked would be the most comfortable option for a kid. Another reason might be that the family lived in a very private place, like on a large farm with no neighbors nearby, so there was no one to be bothered by the kid's nudity.
The whole story of Diary of a Wimpy Kid is about Greg's day-to-day experiences, his dreams and fears, and the funny situations he gets into. It's a relatable tale of a kid trying to find his place in the world.
Well, perhaps there was a situation where two kids were playing near a pool in their backyard. They were so excited to jump in the water quickly that one of them just stripped down right there without thinking much, thinking it was okay to be naked in front of their friend. It was just a spontaneous act of a young child who was more focused on the fun of the pool than on modesty.
Yes, it is highly illegal. Exposing a child to such inappropriate nudity is considered a form of child abuse and a serious violation of the law. There are laws in place to protect children from any form of sexual or inappropriate exposure.
In a sense, it can be considered normal in very young children. At a very early age, kids don't have a fully developed sense of privacy or propriety. They are exploring their bodies and the world around them. However, as they grow older and start to understand social norms, it becomes less and less acceptable.