A1 is restricted, depending on the bike it may need to be restricted. What bike is it?
As for the actual path, I did my A licence right off the bat so can't comment on that one, I believe
@IC3 did the A1 ---> A2 thing so will have better info.
Either way they'll need to pass a theory test before going any further. I *highly* recommend this app for that:
DVSA revision questions & videos for UK motorcycle/motorbike theory test 2024
play.google.com
I didn't do the A1 as to me it seemed pointless, only benefit over the CBT is being able to ride on motorways and ride abroad. I did A2 when I was 19, recently did A (DAS) as a progressive thing. However, a colleague of mine has A1 and is going through the progressive route to his A2. But he needed to be able to ride abroad, so slightly different circumstances.
I would wait till they hit 19 so they can do A2, than after 2 years at 21 they can book the Mod 1 & Mod 2 and do both tests on their own bikes. Mod1 is £15 and Mod2 is £75, going via school will be anything from £490-690, only requirement is for the bike to be above 599cc, there's plenty of restrictable bikes for A2 now-a-days, when I passed my license 7-8 years ago the choice wasn't great... I would probs swerve towards CBR650R, CB650R, MT-07 or Husqvarna 701 SM depending on what type of bikes they like.
I still regret not getting a MT-07 now and doing the Mod1 & Mod2 myself, but after the test I realized that the rideouts with an instructor helped me to prepare for the tests, so the £619 I spent doesn't hurt as much... Just get them to do the progressive as soon, as they can, the longer you leave it, the more bad habits they'll pickup.
Edit:
It is all a big scam when you look deeper into it, its expensive and difficult to get a learner insurance on a motorcycle on a provisional for the duration of the test. So the first test, you are forced to go via a school, my school is good as it doesn't dictate the rules for you, you just want to do the test with them? Fine, but if you fail you, its on you - seems reasonable to me! You need to do a bit of research on your local schools, my mate recently went with a different school, as mine was booked out till the end of the year and to me it sounds like they're ripping him off a little bit (I told him this too). When I first did my A2, I picked training in the morning and test in the afternoon, same with the Mod2 and it was fine for me coming from a 125 which I rode for 3-4 months.
I'm all up for those restrictions, but apply them to all ages, you're 60 and want to ride? A2 restricted and after 2 years it progresses automatically to A, even if I had to pay the £50 admin fee it wouldn't my wallet as much...