I feel the benefit is more personal. Hard to interact socially using a translation device.
When I worked in Netherlands, I found it useful to understand what others were saying to each other, when they assumed I could not understand their comments.

Reading this with interest. I lived and worked in Bulgaria for many years. I thought I was going to spend the rest of my life there. It did not work out that way ! I was well integrated and had no intention of returning to SCO, but I did. "Man proposes and God disposes !"

When it comes to employment certainly a benefit. I have Spanish friends who can't find jobs in Spain and those who speak French, German can walk into a job in Spain. It depends on market but language is still a huge benefit and most businesses want local language support, otherwise they won't buy the product. I work in Ireland and our department currently has 6 positions available. All 6 positions require another European language.

So an Irish speaker who could also speak English, would fit the bill I assume?

Unfortunately that's only in the Public sector. Italian, Dutch, Spanish or French would fit the bill. Just checked now and German always needed it seems.
https://ie.linkedin.com/jobs/german-speaking-jobs

One of the many downsides to this Covid situation is it's stopped a lot of people from getting on with their lives.
The OP states he left a job in the UK that only paid 22k, but would it be easy to find one that even paid that on returning? Problem is that recruitment agencies are not always the best at recognising skills that are developed abroad. If he does want to return, try the jobs market first - but needless to say, may have to wait until the pandemic is over.
Another thing that may be worth working out is the pension side. We know Spain (like other European countries) pays a far bigger state pension than the UK. This obviously depends on contributions made, so maybe investigate the situation. Will he be easily able to keep contributions going or transfer to the UK system?

Some very good points. There are many things to consider as regards the childrens' future. For example, if moving back to a British city, can you avoid them getting caught up in the gang violence (or is it a problem in the current Spanish location)? Do you want them to go onto university? If so, the tuition fees will be enormous back in the UK. Otoh if they are top achievers, may be you want them to try for Oxbridge?
Hard to decide on many things without a crystal ball.

thats the thing, sometimes.. it just doesnt work.

blimey with those salaries i might have to investigate. My other half speaks 4 languages fluently, and can get by in 3 or 4 more , i speak 2 and can get by in a couple more, yet here we are, working for 1000 a month.