Mostly, they are suitable. However, some very young or sensitive children might find certain parts a bit too spooky. But in general, Stine's stories are designed to engage young minds. His stories often have a moral or a lesson at the end, like not to be too greedy or not to mess with things you don't understand, which is great for young readers.
Overall, yes. The short stories are written in a way that young readers can understand. They use simple language and concepts that are relatable to kids. The characters are usually kids themselves who are facing strange situations, which makes it easy for young readers to put themselves in the story and enjoy the adventure while also learning some valuable lessons along the way.