As long as we are talking safety, opening up a 12V lead-acid battery to tap 3 cells if probably not a good idea, especially an originally sealed one.
On the other hand, a 6V DC motor can easily be driven from a 12V supply with 50% duty-cycle, no need for special voltage regulation. If the toy has no electronic circuitry or electronic throttle control, you could simply make a 50% duty-cycle square wave oscillator driving a N-channel low side MOSFET (at 30A a single cheap jelly bean IRFZ44N with 17.5mohm RDS-on would need to dissipate just over 8W @ 50% duty-cycle with a 12V gate voltage) to the motor negative (or what would be the original 6V battery negative terminal if there is a reverse switch). If there is some kind of throttle circuit (which would probably be some rudimentary PWM controller), it would be more complicated, but you would basically need to check if it would support a 12V supply (check capacitor voltage ratings, etc) and find a way to limit the motor switcher's duty-cycle to 50%.