Mahler is one of my favorite composers, but because of the length and intensity of his music, I don't necessarily recommend him for children. The best way for me to listen to Mahler is when I'm all alone, with two hours to spare, so I can just sit and let the music wash over me like a big ocean of sound. (I was able to do this one summer when I was monitoring the listening lab in the David Gardner building at the U. It was awesome.)
There's something about the harmonies, especially in Mahler's slow movements, that feel cathartic to me. The first time I heard the Adagietto from his 5th Symphony, when I was ushering for the Utah Symphony, I felt a huge release of all the inner struggles I was facing at the time, and it came out in tears streaming down my face.
Each symphony is unique, and ginormous--the 2nd, "The Resurrection," has soprano and alto soloists, an off-stage brass ensemble, and a full chorus. There is a mandolin part in the 7th symphony. The 8th is called, "The Symphony of a Thousand," which includes two SATB choirs, a children's choir, and eight soloists along with the orchestra.
I talked about Brahms and Wagner's dispute over programmatic versus absolute music earlier. Mahler strove to bring those two ideas together in his symphonies--he used the symphonic form, as opposed to opera, but brought in extra-musical ideas, literary texts, and especially his own personal experiences (many of which were tragic). Here's a quote I found: "The symphony must be like the world. It must embrace everything."
And all of this from a guy whose instrument was...
Given the kids probably won't sit still for an entire Mahler symphony, I recommend the third movement from his first symphony ("The Titan"). He incorporates a tune from his childhood, "Frère Jacques", but puts it in a minor key, and turns it into a Klezmer funeral march. It's really, really cool. Also, if you do try to listen to other movements, the first symphony features off-stage trumpets; and instructions to the entire horn section (of which there are 7, instead of the usual 4) to stand while playing their final fanfare in the last movement. It's all really, really cool!