Our 6.5 year old son Ryan has listened to the first 35 Magic Tree House books by Mary Pope Osborne. There are 46 so far. I just ordered from the library the next 10. Besides the first one which I read to him, he has listened to them on audio. I recommend them for introducing general knowledge about history and different countries in a tame way suitable for 5-6 year olds. The CD’s are available from the library or you can download a number of them from the library or buy them.
My only caveat is that there is some glorification of mysticism, gnosticism, and native religions in some of the books that if you are a Christian, you are going to want to quash. That is to say Jack and Annie sometimes get out of jams by quasi-religions figures (medicine man, ninja master, etc) advising them to meditate, focus, concentrate, become one with nature, etc. As a Christian, I would say that these tales need to be supplemented or overwhelmed by immersion in the biblical narratives (see my recommendations of Bible story books or NIrV, What’s in the Bible) so that our children can grow in their sense of how the God of Jesus Christ acts in history and develop the ability to detect dubious imitations.
Still, the Magic Tree House books provide a basic working knowledge of people and places from dinosaurs to George Washington to India to Thomas Edison. The kids like it when they can look at the pictures in the books while listening to the audio CD’s–though ours don’t follow along in the books carefully–instead mostly just listening. Our four year old can sort of follow them but I would recommend age 5. The author, Mary Pope Osborne, reads the books and does a good job giving voices to the different characters.
When you have the chance to read out loud to your kids, I would recommend working through some of the kindergarten books recommended by Jim Trelease and buying his guide (see How to choose which books to get at the library or see my categories Books and Library Books). Trelease recommends the first book of the Magic Tree House series but they are not superbly written in the way that Charlotte’s Web is so I would say use the audio for Magic Tree House as almost-educational-TV-entertainment and read to them more quality books with your read aloud time.
My sister-in-law who has older kids said they enjoyed the non-fiction books that can be read as background for the books–the Magic Tree House Fact Trackers but we haven’t gotten into those.
The first 28 titles are short while 35-46 are almost twice as long but you won’t really notice. The latter have the subtitle: “A Merlin Mission.” (Source: Magic Tree House website).
Our local University of North Carolina Morehead Planetarium and Science Center has a Magic Tree House Space Mission Planetarium show.