The Ivy and Bean books are very reminiscent of Beverly Cleary's books centering on Ramona, Beezus and Henry. Bean's 11-year-old sister, Nancy, is uppity and scornful of her little sister, and there's a little less love lost between them than between the Quimby sisters. Bean even looks like Ramona, with short straight hair and a proclivity to make noise and break things. And tease Nancy. Also, girls who read these books will be able to relate to the two main characters; I could see my tomboyish 7-year-old self in Bean, my shyness in Ivy.
It's almost impossible for me to judge by grade what reading level a book should be. I was homeschooled; I only knew what grade I was in so I could explain all this to the clerk at the grocery store when they archly asked why I wasn't in school that day. Having already compared these books to Beverly Cleary, I'd easily put them in the same reading level. And, since Ivy and Bean are in second grade, I'm going to go ahead and assume that that's the grade the book is written for. I enjoyed them, and I think slightly older kids would as well, but not too much older or they'll become boring. The books strongly focus on the active imagination of smallish children, and once someone gets to Nancy's age I am guessing they'd have the same frustration with the pranks and make-believe employed by two seven-year-old girls.