Good idea with the smaller hub, but since Ive got more than one battery, with different voltage, MP best since we could lend from each other. Different hub will give only one battery? And Im guessing its a alu-fork? Isnt that most usuall?
Will the breaking when pedaling backwords have any downsides?
Before deciding on a minimotor, you must first decide if it will be the best choice for your particular requirements.
Here is some useful information which may help you to decide:
Similarities between the minimotors and the Magic Pies Both can be run on 24, 36 or 48 Volts etc.
Both can be used with disc brakes.
(New type minimotors only.) Both have the same 100mm width axle fitting.
Both have cruise control.
Both can use Pedelec
Advantages with the minimotors Minimotors have a freewheel unit inside which virtually eliminates the motor resistance when riding normally without motor assistance.
The built in gearbox greatly increases the torque supplied by the motor.
Minimotors are smaller, cheaper, lighter and much less obvious than the Magic Pies.
(Good Stealth Factor.)Disadvantages with the minimotors Regenerative braking won't work with the minihub because of the freewheel device.
In the event of hall sensor failure, they cannot be used in sensorless mode.
They require a separate external controller to be fitted.
They are slower because of the built in gear reduction.
They are more prone to damage from overheating when worked too hard.
Karen, you can use a magnet to check if the bikes have steel forks, if it sticks to the forks they should be OK to use with a front motor.
As far as the existing rear brake is concerned, the only drawback I can see is that it will not have a power breaking switch, because the rear brake lever provided with the kit cannot be used. If your parents are used to mainly using the rear brake, it could cause a problem when using the cruise control, because when the rear brake is applied, it will not turn off the cruise control. This would result in the motor being unnecessarily strained by working against the rear brake.
Alan 
