Sorry, this is a lot of twaddle.
I live in Spain. Yes there is unemployment but our universities are perfectly good thank you and have the advantage that you don't leave saddled with ridiculous debt because we still have fairly priced universities.
There is absolutely no reason to believe that OP's daughter would be "saddled with minimum wage jobs for life" her father is a teacher. Her education would not be ruined at all, it would be different.
I am currently doing a law degree in Spain. I have friends who are lawyers, notaries, accountants....just like in the UK such roles exist. None of them are scrabbling by on minimum wage.
Separately, Spain has a far superior health system than the NHS so if OPs daughter has children in the future then her care will be better. She will also have access to publicly funded nursery school should she wish to return to work.
Your post reeks of British arrogance and ignorance of Spanish society.
OP to be honest if I were you I would let her go and I would go with her.
Of course once she has been there for 6 months with your consent Spanish jurisdiction will apply to any custody disagreement and 50/50 is the default here, particularly of children of your daughter's age.
I expect that if she came here she would want to stay and her wanting to return to the UK would be very unlikely.