Fitting ebike kit, changing rim diameter?

Assuming the disc rotors are the same size, the brakes would work. However, you'd now have mismatched front and rear wheels, and the geometry of the bike would be changed. The head and seat angles would be slightly more laid back, and the bottom bracket would be a little lower (which could lead to pedal strike when cornering). You would, at minimum, probably want to adjust your saddle position and angle. You'd want to carry two sizes of spare innertubes to be prepared for a puncture.

If the kid is a hub motor that you have to build into a wheel, then its not 26" specific. You probably need different spokes though, the electric motor hubs tend to be a lot wider than whatever was in there.

If its a whole replacement rear wheel, then I guess you could salvage the motor, and build it into a wheel. Again, different length spokes needed, and you'll want to use a good rim, with minimal wear.

The main considerations will be the changes to the gearing. A larger radius wheel will have less torque than the specced 26" wheel, but a higher top speed because when the motor is at max RPM, the larger wheel will roll further for each revolution.

Minor things like length of wiring harness, OLD of motor vs your frame, may also be issues on the way.