Do I need to lump it? Or is my job taking the piss?

[JokersToTheLeftOfMeClownsToTheRight]

Just feeling really down about work - and wanted to get some advice!
After a degree and good-ish career, I didn't work for a long time while I had kids. As a way of getting back into work, I have recently taken on a job as the Manager of our village community shop. I was told it would be 8-2pm, 3 days a week (I also work somewhere else the other 2 days and have three kids, and my husband works away from home during the week - so I am busy!).
I am being paid minimum wage - and yet I am doing everything. All accounts, ordering, managing around 30 volunteers, managing two paid staff. I get called all the time on my days off to sort out problems (including during the lunch break of my other job) . And there is no one else in charge - so I feel the weight of the responsibility of making sure everything runs ok.
I was happy to take a minimum wage job (even though money is tight) - because I thought it would fit around my kids. BUT it is the opposite. I am working way over my hours. This week I had to leave my kid home alone while off school sick (not anything bad!) because I had too much to do at work. And I am leaving my kids to go in once a week in the evening from 7-9pm to cash up. And often in again at 6am before the kids are up. As there is just too much work to do! Plus I never finish until 3 / 4pm.
I am waking in the night, and early mornings - feeling stressed out. All the volunteers are lovely, and very supportive. But one of the other assistant manager (old man, been there for years, doesn't like change) is being really difficult - and making things unpleasant.
So my dilemma... Do I put up with this, because it will look good on my CV? I do need the money! Am I being too sensitive? Are all jobs hard work - and I just need to get on with it? Or is this a crazy amount of stress for a minimum wage job?
They do pay me for the extra hours I do - but that doesn't really compensate for the (unexpected) time I am loosing with my family while at work.
Sorry for such a long, boring post. But I am feeling very emotional, and in need of impartial advice. So please be kind!!

I worked in a high level fashion retail. We had computers instead of tills and various software, so we produced reports and tracked sales. We were constantly told to make more and more sales and given stupid targets to achieve. No end of the list of duties, all this for NMW

according to me this was a job for at least 17 p/h but anyway

You need to speak to the board (or whoever your manager is?) and lay out the situation for them. They either need to increase your hours or decrease your workload. From here on, only work your contracted hours, don't be available outside of work or explain if you do need to be then you need to be paid for it.
You need to have a clear job spec or at least task list. Roughly calculate how long each task tasks and then work out what your capacity is. If it's above your contraced hours, they pay you, it doesn't get done, or someone else does it.

Or leave...

What age are your children?

Delegate stuff to the assistant managers. Ensure that when you are working, you are working as a manager, and not manning the shop. Tell all the volunteers that if they are unable to do their shifts, then they need to contact (male assistant manager) and make him responsible for finding cover. No cover, shop stays closed due to staff shortages.

Thanks to everyone who replied - I really appreciate it! It helps give me some perspective!!
Just to answer some questions....
I am paid just over minimum wage (£13.20) and assistant managers are paid £12.70. (as of this month since min wage went up!) So a slight difference. BUT I am basically responsible for pretty much everything - and they only do alternate weeks for 2 days a week. I have tried to talk to them about taking on more - but got a very passive aggressive/ difficult response!
I don't do any till work - and still can't fit everything into my hours. So it's not like that can change.
It's run by a hands-off committee who meets about once every 4 months. So I don't really have a 'boss' to help me.
I think people are right - I need some boundaries! Or I need to leave!
My kids are 15, 12 and 9. We live in a village - so almost every night one of them needs a lift to some sort of activity. Which I am happy to do! And would be manageable under normal circumstances - but is exhausting with all the extra work hours I am doing.