Hello babymum,
When you open your Little Math, you'll see buttons at the top, "Home" "Playback" "Numbers Library" etc., so click on "Playback", then look for the Playback settings right at the bottom. Put "Override" on by clicking the button, (green light means its on.) Next to "Override" there's a settings icon, click on it to bring up the settings menu. On the settings menu click on "Numbers" tab, the second panel on the numbers tab is "Icon", there you can click on the button next to "Icon Set", the light there should be green too, and then select the option you want from the drop down list. Remember to save, the save button is at the bottom right corner of the settings window.
Hope this helps, maybe I over explained a bit, but I just wanted to cover everything I thought you might be missing.
I have to add new icons all the time or else my little one is just not interested
Cheers