Have you tried a kneeboard? Someone (HO I think) makes a kneeboard with a collapsing handle hook. to start you put the handle in the hook and the rider holds on to the board. Once up they can fidget with the knee strap and grab the handle. We have an O-Brian Black Magic with a non-retractable handle hook. My kids friends get up on the first up, but the handle hook doesnít retract and once caught again when as the rider fell, dragging the rider for a few moments, kind of scary. Get the kind with the collapsing hook!
Iíve found that the kneeboard is a good stepping stone, especially for the young.