There are now close to 30 trumpet duets and trios on the TrumpetFish site, with corresponding sound files there for most of them. These are mostly geared toward players with 2-4 years of experience, but a few of the later ones in the table start to be a bit of a challenge. They're free to download, so give 'em a try!
There are also links/pages that include trumpeting ideas, music, and on-line lessons for the younger set. Topics range from breathing to counting and jazz. It's all free at TrumpetFish.