Op I think YAB a tad U! My sister completely retrained and changed her career at 37 and she had fewer adaptable qualifications and experience than you.
Did you get a good law degree? If so, why not do a Masters in some technical area of law and then try and lecture in some obscure aspect of law? That would be still very demanding but more family friendly in terms of holidays.
The thing is, hindsight is a wonderful thing, you've made your decisions and you can't go back, so there is no point in fretting or comparing your life to others, just own the fact that you have travelled and lived abroad and have experienced and trained for another career whole others were slogging in a law office!
You are in a slightly odd position because a lot of people put in all the long hours and stress early on before having dc late. But you have recognised that that life doesn't suit you, and you are academically bright, so why not try and use that in some way? The trouble is, apart from the holidays, universities are extremely stressful places to work nowadays, so you have to be totally committed to it. And if you didn't want to be promoted when you teaching because of the extra workload, then university life may not be for you either.
Put very bluntly, it's not very logical to quit working in a law firm because of the long hours and stress, and then avoid being promoted in a school because of long hours and stress, and then complain that you don't have a high flying career! On the same grounds, it's illogical to think of yourself as a failure when you chose the path you did. You haven't "failed" - you chose a different path.
The important thing now is to be very clear about what you do want and maybe see a good careers advisor or mentor? And make your past qualifications and experience really work for you.
Maybe working for yourself would suit you better? A high proportion of teachers and lawyers go on to work for themselves so your background is perfect! The key thing now, whatever you choose to do, is to be committed and not to dither.
What about setting up a tutoring service for children? The problem with that of course is that you are needed after school hours and weekends when the dc need you. Someone I know has set up a proper after school homework club in office premises, she employs staff, and provides group and one to one tutoring and meals for older dc and bridges the gap between school and home from 3.45 to 7.15pm. It's not cheap!
Or would you derive satisfaction from teaching sc with special and additional needs?
I'm in my mid-fifties and people achieve their individual version of "success" in a myriad of ways and by many different routes! Someone I know in their late fifties is setting up a small business right now!
Good luck op! The world is your oyster! 