A "fun" piece for a junior orchestra winter concert, based on Good King Wenceslas including a section that can be opened up for improvisations. It's deliberately using lots of jazz clichés that children will find fun and easy to play.
When we did this we first learned all the melody rhythms and phrasing vocally, as most children were new to jazz. This way, when they came to reading the simple rhythms, they were already phrasing them together. We also reviewed the sharps for the Bb and Eb instruments (key always being tricky for beginner orchestras so that strings and winds have relatively easy notes to play). We also were careful to get the dynamics right. It was pretty easy to get together. We noticed how the melody was played by different sections of the orchestra.
To get into improvising, we spent some time echoing "jazz" rhythms using any of the improvisation notes given at the end of each part so that the children would be eased into improvising while getting used to notes that would work (no one was worried about "wrong notes").
For the concert, when we got to the improvisation section all the children wanting to do a short improvsation stood up and played a short phrase each. You could have each soloist point to the next one to free you up as a conductor to bring in the backings (which you can leave out to start with then bring in, stacking them up), or have a section leader "in charge" of bringing in their own backings. I thought lots of children would be too shy to improvise in the concert but we had about half the orchestra! Some had prepared a phrase to play, some improvised, some played some completely out stuff - all of this is absolutely fine! Lots of children are scared of improvising so it is very important not to criticise them - let them find their own way and learn from listening to others (teachers and students alike).
There are PDFs of the score, violins (1, 2 and 3), viola, flute (violin 2 also works for beginner flutes), clarinets (1 (no register key involved) and 2), alto sax, trumpet, trombone & piano. If you need extra parts you can probably easily adapt the existing parts. There is an mp3 with absolutely dreadful computer generated instrument sounds!