Children’s/Middle Grade Sci-Fi Book

Trying to recollect a book that I started as a 8-10 year old child in the mid-2000s. I read roughly 1 chapter and had to return it to the library which has made it difficult to track down.

The only thing I vividly remember is the cover which contained a human child (I think it was a boy but could have been a girl) and their floating alien companion (reminds me of the green guy from the Hitchhiker’s cover) over a backdrop of an alien city.

Obviously not a lot to work with but always wanted to find and finish it and know that I have kids, want to take advantage of this second chance.