Really it's not your fault, the jobs market is TERRIBLE at the moment. I have a degree but am doing a very basic poorly paid admin job in a school (part-time) after staying at home with the kids for 4 years. I found out that 150 people went for that little part-time post, most of them very highly qualified with degrees. I think I was just very lucky.
In our local paper which used to have pages and pages of jobs, this has now dwindled to a couple of pages each week, and I'm in a big city. The professional jobs paper which I used to get aimed specifically at people with my degree is the same - used to show loads of jobs all over the country, and I had a look at it recently out of curiosity after a few years of not bothering and there were only 3 jobs in the whole country for that fortnight's listings. I was truly shocked.
Things are really bad at the moment. Really bad. So take heart that it's no reflection on you and your abilities, it's just the way it is at the moment.
As others have said, who's to say that the job you originally had would still be there anyway in this climate? The problem is, no-one who gave up their job a few years ago could possibly have foreseen the jobs market getting this bad.
If I were you I would try and feel positive that you have done the best thing for your children. Keep on with the voluntary work and know that you are doing some good there, when voluntary organisations are struggling at the moment too. Keep on with the job applications,and definitely call them to get feedback - the fact that you ring to follow up will show that you're more interested in someone else who is just appyling for any old job.
Could you try and do a college course that isn't 3 or 4 years long? Just to keep your brain ticking over? It could also open a few doors in terms of socialising with new friends etc.
Fingers crossed you get sorted with something that is right for you very soon.