Posted: Mon Sep 7, 2020 10:32am
I've done a bit of googling and I hope this site will make it easier for you to look at the options as they currently stand for non European nationals. We can only hope that the uk and Spain may make a more user friendly agreement in the future!
Basically, the non lucrative visa would cover your mum as a retiree with sufficient means, but the issue will be for both you and your son will be how you go about satisfying any criteria for work visas. If you have skills that Spain needs then you may be able to obtain a work visa, and it's very possible, I think, that some form of working holiday visa scheme will come along for younger people next year. Before we joined the EU such exchange schemes were commonplace, but of course they all disappeared with EU freedom of movement as they were no longer needed.
Hopefully, this link will take you to the right page for the non lucrative visa, and you can navigate from there to the work visa pages for a bit of research.
The problem really is that the process for uk nationals simply doesn't exist in practice at the moment, as we are still in the transitional period. I truly hope that there will be a mutual agreement between the uk and Europe to make it easier for everyone post Brexit, but at the moment there isn't anything agreed so far as I'm aware.
https://visaguide.world/europe/spain-visa/residence/