The 1.5 and 1.2 volt figures are only nominal anyway - new dry or fully charged cells will give a higher figure.  An important difference is that normally, a rechargeable cell can give out a LOT more current than a dry cell, but its maximum output will not last as long.<div>A toy designed for use with dry cells is unlikely to be able to demand the level of current that would cause damage, even with a couple of extra cells to bring it up to around the nominal 9 volts.  On the other hand, the extra weight might not do you any favours.</div>