I think a lot would depend on the individual.
There is a lot of discussion about push bikes for kids learning to ride. Take the pedals, cranks, & etc off, and just get them to learn to push along with the feet and coast.
Of course, the kids could get too used to using the feet on the ground.
It isn't the same as riding a bike, but I thought it helped my nephew to ride a trailer bike. No control, but some feeling of balance, as well as pedaling practice. The Weehoo bike is a recumbent style, and perhaps gives the kid a little more support.
There is also a KIDZ Tandem. The advantage of that configuration is that it puts the kid up front on the tandem so they can be better monitored by the adult.
There are also recumbent tandems or combination recumbent tandem in the front, and upright bike in the rear.
For bicycles, you might look at several.
Foot Forward
Semi Recumbent
Recumbent
They all get the person lower to the ground, and with the foot forward and semi recumbent, it is easier to adjust the seat to a level where one can get the feet flat on the ground while on the seat.
The recumbent and semi recumbent bikes also tend to have a bit more supportive of seats (if desired).
I'd probably chose pedal brakes (coaster brakes) over hand brakes. Several internal gear hubs can also support pedal brakes.