I hate to bring bad news, but this is not as easy as it probably seems. Let me explain some things about your typical inexpensive Lead-Acid battery charger. First, they usually have no filtering at all and do not provide a very good power source. Many employ only a 1/2 wave rectifier, a series power resistor, or just rely on the internal resistance of their inexpensive transformers. They just are not designed to directly run a motor. A charger need only have a slightly higher "peak" voltage than the 6V battery, maybe 7.5 V under load, but to drive a pure resistive load or motor with no battery, its effective or "average" voltage may only be about .318 of that, which amounts to about 2.4V (.319 x 7.5, assumed). To make matters worse, that 1/2 wave rectified DC is a pulsing voltage, which is more like AC in many respects. A form of pulsating DC called PWM (pulse width modulation) would normally be a good thing for your purpose, since this is the way speed is controlled in those toy cars. But that is done with an electronic circuit that can control the pulse width. Your pulsing DC from the charger cannot be easily controlled.
So two approaches. One thing you can do is start with a combination of the charger and an actual 6V battery, probably a gell-cell. It may not need to be as high a capacity as the one in the powerwheels, but maybe something small like a 4 Amp-Hour battery. The combination of the charger and battery will result in a more steady DC voltage, similar to the way your car constantly charges your battery with pulsing voltage from your alternator, so the battery can then deliver nice "motor friendly" DC to electric motors in your car (wipers, blowers, etc). And now you have a battery that is charging, at least somewhat, so the total run time may be significantly higher.
That will help get your motor running. But now you have to "concoct" a speed control, and that's where it gets more difficult than you may have anticipated. if you had more time (you only have about a week now) you might be able to find a REAL PWM speed control from another "power-wheels" (if it has speed control), that is ready to be thrown away. Perhaps an "as is" one from a thrift store. The truth is, most of those toys are discarded when the battery will no longer hold a charge, and the parents see how much batteries can cost.
Another approach, if you do get a heftier charger (or maybe yours on the higher voltage setting) and it gets the motor moving without the added battery, AND... you just need it a little slower, you could add series resistance between the charger and the motor. A series resistor as a speed control, will have several disadvantages. It won't give you steady speed under varying loads and worse, it also might run hot (meaning a burn or fire hazard!) Finally, such a power resistor won't be something you can pick up at a local radio shack, and even if you could it would be impossible to know the ohmic value you needed. If I had to come up with such a resistor in a hurry, I'd go to a thrift store and get an old toaster, or something cheap with coil type heating elements. Such heating coils are made of resistive Ni-chrome wire, and are usually a pretty low resistance. With some heavy duty clip leads, you could play around with adding series resistance from one of the coils units until you find a length that will slow the motor to the speed you want. It will also quickly reveal how slow you can make it before it stalls.
Now honestly, I almost am afraid to suggest the above. It sounds temptingly easy but don't forget... a resistor you make from a heating element might get hot. Hot enough to burn somebody or worse, start a FIRE!!!!! It all depends on how long a length of heating coil your ideal resistance is spread over, and it would be wise to keep that heating coil in the appliance rather than tying to re-enclose it in your project box. Otherwise, you could end up with a much scarier Halloween display, especially if it starts a fire or burns someone in your house!
Anyway, I've rambled long enough, and I'm sorry if I've discouraged you in doing this project. On the other hand, maybe you'll be able to get something useful out of this and that you make sure to be safe and sensible if you try anything like I've suggested. Good luck! :-)