I feel your pain my son had a balance bike from 2 years was great on it switched to a bike with pedals at 3 and showed no interest till he was 6, now 7 and on a 24 inch bike that he won’t put down, trying to master wheelies at the minute.
Anyway try the pedal magic random rock I think there is a video online, essentially take stablisers off the bike, sit child on bike feet on pedals, whilst holding the bike tilt from side to side randomly, get the child to steer into the tilt as they instinctively turn the handlebars in the opposite direction which will put them over the bars.
Once they can do this 10-20 time correctly go find a big empty car park let them know the faster they pedal the easier it is to stay upright.
I’ve helped plenty of kids using this method 10-20 minutes and there away.
Good luck you will not wipe the smile off your face for a week once they get it.