Well, to answer your initial question, each LHOTP book has a different level, but most are in the fourth grade range. Little House in the Big Woods is a 4.2 with a Lexile of 930 and Guided Reading level Q. Most of the books are similarly leveled. I read Little House on the Prairie with some of my students last year, and they did need support. It has a lot of historical references that students at this age may not understand, and the vocabulary can be difficult. If she is a struggling reader, you may need to make this a "reading together" series. (But, I can't say for sure because I don't know how much she struggles.)
Have your DS give the Boxcar Children books a chance. He may really enjoy them...for him, they are historical,
I don't really "do" series books with my kids, but I do notice that many of my girls like the American Girl books (RL from 3.5-4.2ish) and Cam Jansen (2.2-3.2ish). Boys like (of course) the Wimpy Kid books and Weird School (actually, so do the girls).
I use the Scholastic Book Wizard to find book levels when I am unsure. You can have it level using Lexile, GLE, or Guided Reading level.