I agree there should be more time spent on gravel /dirt roads, whilst a p plater is learning, but again it depends on a lot a factors.
Time and experience is a priceless asset, which parents can pass onto their learner driver apprentices, in between the rebutals and the back chat from the know it all learner driver.
I have found that simulated computer games, ie rally games at timezone, is a good foundation, there is some feedback co-orindation between changing gears, braking and steering, but they will get the general idea of speed and dirt racing. The added bonus is that they are seated and not powering down the road a breakneak speed.
I put my 1st daughter through 10 hours of that rally games at timezone over 12 months, cost about $200 to $300 in coin, but it was fun.
There is a technique to driving on dirt roads, obviously lowering speed helps, but watching for diffent cambers on corners, loose centre gravel, tyre ruts, clay washouts and greasy and wet surface.
Over the years of being involved with amateur rallying on dirt/gravel roads, the best advice to any new driver, is don't keep the vehicle stable like you drive on the black top, always try to keep the vehicle in a minor unbalanced state, which means always giving minor corrections into the steering, similar to driving a old HR or a tractor with the wobbly steering.
You have to drive on gravel/dirt roads by the feel of the vehicle under your butt, it sounds wierd but after some experience, you will generally get the hang of it, with higher 4x4, this even more important and if travelling with a trailer on behind, even more important still.