Hi Olle,
It sounds as if you (or her!
) have managed to select a MIDI template which is expecting the style bass note on a specific MIDI channel ('MIDI Pedal 1' or 'MIDI Pedal 2' for example). If this is the case, and there is nothing connected to the keyboard that is sending notes on that channel, then the keyboard will just use the same bass note for all chords.
On a S950 you can check this by pressing FUNCTION > MIDI. The MIDI template in use will be highlighted.
Unless you have any specific MIDI requirements, the simplest thing to do is to select the preset 'ALL PARTS' template, which should restore normal style play.
You can read about selecting and editing MIDI templates in the section 'MIDI Settings' starting on page 104 of the 'PSR-S950/S750 Reference Manual' (english version). Note that this is not the same as the 'Owner's Manual' which you may have received with the keyboard. You can download the Reference Manual from your local Yamaha Support website.
p.s. next time don't try and blame the baby!
Regards
Ian