Children cannot stay focus in a classical concert especially when a long long piece is being performed - that is because they don't have enough information to interpret what's in the music.
This make introducing classical music to young children extremely difficult:
There are two rules I stick to when I select a piece of classical music:
1) the is SO interesting that children can create a story to it just by listening. (Recommendation: Mussorgsky's "Pictures at an Exhibition")
2) I can find visual aids that can accompany the song.
This video is an excellent example of letting children SEE music. It's one of Bach's Toccata played on an organ. Time duration is over 8 minutes long.
Even Year 1 can sit through the whole song without losing focus for a second. And they even asked for a 2nd playing.
Perhaps you would also like to see how Disney interpret this piece. This following video is from the Fantasia 1940.
Another site I HIGHLY recommend is from Netherland. The creator of this site cleverly designed the animation to fit the musical elements of some famous pieces. Long movements are cut short, so it doesn't take a lot of time to go through each animation.