Op I sort of understand why you bought the present. I too have MH problems and sometimes our desire to be the provider of great presents is clouded. I've been in a similar situation though I did have plenty of food in the house.
That's the part I'm struggling with, what are you going to feed your 4 children ( and yourself ) for the next few days?
From my own experience, the kids I know who hardly get anything are the most grateful. The ones who get a lot ( mine included ) can often take it for granted or expect it so it's not a big deal to them.
My own ds is 15 and he has everything under the sun tech wise, his dad and I are separated and he's very well off.
My ds knows that I can't buy him what his dad does, my priorities are living costs first and foremost and I don't go without things myself just so I can buy him things he doesn't need.
It's tough as I only have 2dc, I couldn't even imagine providing for 4 and in a cramped house. Are you able to look at moving to a larger property if at all possible?
My ds has AS and he works with his dad now and again as I don't want him having to work yet, he's in an important stage in his school life with exams next year and he needs to concentrate on them.
It's easy for people to criticise you without knowing the full extent of your situation and I think your heart was in the right place, please don't be too hard on yourself.
Hope your situation improves going forward, you've been through a lot and deserve to be happy 