The children's librarian at our public library is fantastic and puts together these "Grab and Go Bundles" for various reading levels. It's been a great way to get my son to try some new series etc because we just check out the 5 or 7 books in the bundle and read them all.
I mention that to say... talk to the children's librarian. Don't underestimate the amazing knowledge those (usually) women have. They have degrees in children's literature and they know so many titles and authors and levels.
My son is a bit beyond your kids' reading level, but he still loves the "Elephant and Piggie" books. They're easy to read and he thinks they're hysterical and always checks out two or three :) There are many books in that series, which can be fun.
For picture books, Knuffle Bunny, is pretty cute and has a few books in the series.
The Frog and Toad books have also been big favorites in our house.