You do not note whether you and your grandson are working with RAW or JPEG images. As noted, GIMP is excellent for JPEG, but it does have a different layout and working process. YouTube does have many excellent videos on using it.

If you are wanting to edit a RAW image, I recommend DarkTable, there are a few others. It is free, open source, and frequently updated with a major upgrade at the first of this year. With that too I strongly recommend YouTube, specifically Bruce Williams (but there are others), to learn how to use it. The manual tells you what it can do, but not how to do it.

Big difference between open source and proprietary software is that everyone writes a book on how to use proprietary software and hence earn money off of it. Those that publish to open source typically do it for free and earn money from the graces of those that want to donate.

I recommend using a USB card reader as it is faster than trying to transfer files from the camera to the computer via a USB cable. Side note, Linux is faster at it than Windows.
Make sure that the USB card reader will support the higher capacity cards. Easy solution is to by one that supports USB 3.

To edit RAW images for free use Darktable. Suggest that you review videos on YouTube by Bruce Williams and others. The free manual is appropriate, like around 250 pages, but you will not find published books on it by those that want to make money. The manual does not tell you how to do it, just what it takes to do it. You can do full editing of your RAW images and then export them to whatever format you desire. It is a very powerful program with many options that takes time to learn but it is excellent.

Are you looking for storage at home or on the Internet?
If at home, buy a dual / quad external hard drive box and appropriate number of hard drives, 1, 2, or 3 TB size, that connects to your computer via a USB cable. Buy one that supports raid for protection.
If going onto the Internet, I recommend a service called Forever. As I understand it has a one time cost (not the cheapest and varies by the amount of storage that you want) but they do backups and do not share or take ownership of your photos. You can designate others that are allowed access as it is not accessible by the public.
A major issue is what format you are storing your photos as, RAW, JPEG, or other. You can store RAW on any sited but it generally takes up more space.
Another issue to be concerned about is storing your photos on some site that can then claim ownership to them and use them for their use and although you have access, they have rights to use with no benefit to you.
I am sure there are multiple other storage options, my suggestions are limited. Look forward to other suggestions.

Look into DarkTable. It is excellent software, is Opensource and FREE. Some believe it to be better than Lightroom in that it has a larger number of modules to work with. Suggest that you view Bruce Williams on YouTube for many user documents on how to use it. Yes, it takes a lot of work to learn everything that it can do, but once mastered you will do well.

Why is everyone hung up on proprietary software that costs $. Look into Darktable which is FREE, Opensource, and can run on Windows, Mac, and Linux. There are a number of excellent videos on using it, I recommend Bruce Williams on YouTube for a start. Darktable is excellent software and have observed on some videos that it is better than Lightroom. The one downside is the manual, which is over 250 pages long, is written by those that match my modified cliche, 'Intuitively obvious to the most casual observer, when you are intimately familiar with the product'. Actually the commercial software falls into that category too, just that there are many authors that like to write books and make money on the side showing you how to use the software.

For Linux users, the Command Line tool is 'exiftool'. You can both display and edit the data. You can add a 'UserComment' (syntax?) to the data that can detail the information that you desire. I have been given the impression that it can also be run on a Windows system but not sure. When I do add this, I add to the filename '(EXIF)' to let me remember that additional data has been added. Windows will allow the display of the EXIF data under the file properties Detail tab, but I do not believe you can change the data. As for size of the comment, I do not believe there is a limit.

There are several issues in keeping information on a public cloud (iTunes in your case). One of the most important is the cost. Today a one Terabyte drive is around $75, probably less since I haven't looked at prices lately. Realize that there are several different models and the better ones can cost more. Evaluate how much you are paying on a yearly basis to that of external hard drives.

You can purchase a dual external USB drive case and set it up for RAID to automatically back up your images. Some external cases can also be set up as Network Addressed Storage (NAS) and stored in a remote location so that it can be accessed by everyone on your personal network, it is a higher cost.

The one issue that is very important to me is that images uploaded to many cloud servers no longer belongs to you exclusively and the owner of the server (iTunes / Google / ...) can claim ownership of your photos and use them at their desire without giving credit to you or asking permission.

You might also look into Forever, which is a cloud service but provides exclusively private storage and does not take ownership of your images.

This is not a recommendation on what to do but hopefully to help you evaluate what you can do.

I too have been looking into obtaining an extension tube for my Nikon D5600 and have found several brands that I can see have the contacts that support the AF requirement. In looking at the specs and photos I am concerned about the aperture control, as on the 5600, not sure of other models, it is a mechanical control. Can not tell if the lens aperture can still be controlled from the camera. Would appreciate any insight.

Thank you.