This is to be used for demonstrating to children the effect different instruments have on a piece of music, would prefer a classical piece, but essentially something that starts simple and builds as it introduces a new instrument at different stages. Any ideas?
Benjamin Britten's The Young Person's Guide to the Orchestra sounds like an ideal match!
'Instruments of the Orchestra' is one slant. 'Peter and the Wolf' is another.
Good composers generally avoid the pitfall of just continually layering further instruments on top of each other until the result is a muddy mess (though loop-based sequencer jockeys often fall straight in). You could use Britten's 'Young Person's Guide' to demonstrate different approaches to broadly the same musical content featuring different instruments, culminating with the orchestral fugue where they all come together. Or to illustrate different scoring densities of the SAME material (which I think is more what you were asking for) you could find versions of the same piece arranged for e.g piano, string quartet, small orchestra, brass band, wind band, full orchestra. The British or American anthems are available played by a wide range of ensembles. Played back-to-back they could illustrate your point.