I have a toddler size duvet and duet cover that I purchased at IKEA.
Honestly, I'm not sure if the filling is down or cotton/poly, but I do
know that Ikea sells bigger ones in down (twin size might be better
for your child anyway). For the poster asking where to buy duvet
covers, IKEA has a good variety in all sizes, and the prices are
reasonable. Additionally, keep in mind that if you find a 'comforter'
that is meant to be used along, you can just put that inside a duvet
cover of your choice...and you can often find ugly ones at a very
reduced price, so this makes it an even better option!!