As a previous classical pianist and music teacher (I've also had art training) I'm very familiar with the works of Debussy. I've even played many of them myself. The piano works of Debussy like Children's Corner and the first book of Preludes are good choices to teach your child about Impressionist music. But because these are piano pieces it's good to also include some orchestral works like Prelude to the Afternoon of a Faun so your child doesn't think Impressionist composers only created piano music.
Now you have an idea about which music to use don't make the common mistake of getting your child to sit quietly while you play these long music pieces to them. If they want to sit in a bean bag and listen that is fine of course. But most children would rather dance to music. Luckily dancing to Impressionist music is a great way to help children learn what Impressionism is all about. If you use comparison and contrast by letting them also dance to music by non-Impressionist composers like J. S Bach, Mozart, Chopin and Bartok you'll help them learn what makes Impressionist music special.
Another good way is to get your child to create Impressionist inspired art while listening to Impressionist music. This can help reinforce the art activity by showing them how the art and music go together.
But the most important tip I can give is to explain Impressionism to children through fun activities.
Picture: Debussy at the piano