Just a quick point - I know the OP has mentioned ADHD, and obviously comments should be about that, but it's worth pointing out that ADD is similar, but without the H (hyperactivity). Some children will have that variant, and it's worth parents/grandparents knowing more about it.
I have no problem sitting at a computer, whether for work, study or posting on here. I sometimes find I am losing concentration, so play a finding game or similar for a few minutes to refocus, but I don't find study difficult. My biggest issues are with what manifests as procrastination, and with getting deeply focused on something. This makes deadlines tricky, but I always meet them. I just do it last minute, working 24/7 in the last couple of days. If your child is like that, there is no point in nagging them to do Thursday's homework on Monday as they have time - it will probably upset them, and won't make any difference to the outcome. All that will be achieved is that they could end up feeling bad and having the idea that they are lazy or disorganised - and the homework may well suffer as a result. Apparently it's not true procrastination (hard for me to know as I've never had anything to compare against), but more a feeling of being frozen and incapable of making a start until it's absolutely necessary. There is always the black cloud too, as I know a deadline is looming, and I know that I 'should have' done it earlier to prevent the last minute stress, but someone pointing this out is not helping at all - quite the reverse. I know it will get done, whatever they think, and I know it will be done well, just in a slightly different way from how the 'well meaning' types would have done it. I am me, and they are them, however, so it's not about one size fits all. My childhood would have been happier if adults around me had realised this and not tried to change something that couldn't be helped.
The deep focus is less of a problem, but can be annoying when I want to know everything about a topic and let it exclude other things. A child might spend way over homework time on dinosaurs, but not enough on oceans or whatever. Whilst that may even out over time, it's not going to help if they are working to a curriculum. I'm not sure what the answer is if the topics are entirely unconnected, but it's worth being aware. I would also suggest not buying expensive equipment and so on until you are sure that the interest will sustain. I'm not saying that none will - I have interests that I've had for decades - but that they might try many on for size before finding the right one. All children do that, but those with ADD typically get far more involved far more quickly than usual.
Anyway, that's just my perspective, and some of that may not apply to ADHD, but I thought I'd put it out there in case it helps someone.