I absolutely consumed Enid Blyton at that age, and Mary Poppins, and the Borrowers. Not sure when I read swallows and Amazons - some of it definitely went over my head the first time.
Nancy Drew or alternatively the Hardy Boys are also options.
Just William stories as well, and possibly the Jennings books though I am not sure how old I was when I got into them.
In other things that probably went over my head, I started on Sherlock Holmes around the age of 8 and enjoyed them immensely.
I also just learnt to put up with incredibly tiny text but I am also now horrendously short sighted so maybe that was an error. Massively increases the range of available books though.
What about the early Artemis Fowl books? I feel like later on they might be a bit much but the first few should be okay. Also worth considering the H.I.V.E stories - they're all teenagers in the stories but there isn't much romance etc which can sometimes push the acceptable age of a book up. There are some deaths of good characters in them, I can't remember which books.
The Mysterious Benedict Society might be a good one as well, there is a series of those I think.
I think there was also a series about a girl who could hypnotise people, maybe called Molly Moon or Mollie Moon.
I also read and loved all the Dick King Smith books, with a particular fondness for the book about a talking hen called Pretty Polly I think.
On Polly, Clever Polly and the Stupid Wolf and the Milly Molly Mandy books and the Mrs Pepperpot books were also great.
The books I have listed would cover quite a range of ages and some will be much more challenging than others but I consumed books absolutely voraciously as a child and I am firmly of the belief that different books, and different difficulties suit different days.
I still have days where I want to go back and read children's books, I have days when I want to read complex and interesting science books and I have days when I want a chilled out murder mystery.
The key for me is to keep reading and to read widely and some days one might want a more grown up book, others less so. I really struggled with the fact that the books I was physically capable of reading were often much darker than I liked (though interestingly murder mysteries have never really fallen into the dark category, while family misery definitely did).