The Amelia graphic novels - cartoony stuff about Amelia the tomboy - just right for age seven or eight.
The Worst Witch series (written before Harry Potter and definitely for a younger age group.) Fine for age seven.
This is the font size.
And a bit easier are the A to Z Mysteries..
These are great value, which is just as well and they are flown through.
We love Henry Huggins, but his pal Beezus is even more popular. Although you probably already have her.
If not, this is the font size.
I gave this to my eight year old last Christmas - he loved it.
This is what it looks like.
Also, I couldn't find our copy of All of a Kind Family, but that is PERFECT for a seven year old girl. And, now that I think of if the Little House on the Prairie books and of course Pippi Longstocking.