Entitled? No, you're not "entitled" to a refund. You can ask for it though, when your motion is disposed of and you should get it. Motions to modify (or terminate) support can be made effective retroactively back to the date of filing (or emancipation) and that should allow for a refund.
You may not be entitled to a refund but you certainly can make the argument, especially if you never withdrew your motion for reduction. The decision by the court will be based in part upon how you worded your original motion and the subsequent order that was entered. If it is not a lot of money, you might wish to consider it a gift for the benefit of your child. If it is significant. you should have an attorney help you.
Keep in mind that if you are now obligated for 1 child, the reduction will not be all that much as the guideline difference between payment for 1 child and 2 is only 8%.