According to https://www.casio-intl.com/asia/en/emi/products/cdp230r/spec/ the unit has 48 notes maximum polyphony which I can only assume applies to both the keyboard buttons and the sound engine inside. So all the notes should sound. (Some very low end keyboards might only be able to play one or two or three notes at the same time. Yours can do much better so that shouldn't be the problem here.)
Does the PIANO/ORGAN button toggle to a better sound? There are many more piano sound options available through the TONE button as well. They are listed in the "Appendix" PDF available https://support.casio.com/en/manual/manualfile.php?cid=008006005 if you no longer have the manual.
Your keyboard you have does a have many other settings that could be affecting the sound. If your daughter can explain a little more what is wrong with the sound she hears, that might help us troubleshoot better.