Regardless of the people giving your mum a lift turning up early, if she was responsible for childcare, she should stay and look after the children and make that her priority - you don't go off and leave the children alone.
The deal was that there would be an adult present - the Granny and then the Grandad - the Granny went off,leaving g the children without an adult present and that is where she went wrong. It doesn't matter that she had a funeral to go to or anything else she wanted to do - if she had said she would look after the childrn or ensure the Grandad did, that should have been her priority.
The age if the children is irrelevant too - because she told her daughter that the children would be looked after, but then reneged on the agreement.
I would say that you cannot rely on this person to carry out childcare. It is a real shame as having family do it is often great and of course saves money. OPS, I'm afraid that however convenient and tempting it might be to give your Mum another chance, because it will save you money and be convenient, you really shouldn't, because she has shown herself to be unreliable.....and sadly you are not entirely surprised.
Disappointing...but better to know now and to be able to sort out alternative arrangements.
I wouldn't go into a fury about it. I would say quite simply, that you had left her in charge of either being the adult present, or arranging for the Grandfatehr to be present, but she failed to do what she agreed to, so you cannot leave the children with her again.
This is all assuming that there WAS a clear agreement that Granny would be in charge and handover to Grandpa. If actually there wasn't a clear agreement about it all, then I still think the Granny was wrong to leave the children, but the OP needs to take some responsibility too for leaving the children in to situation without a crystal clear agreement about what was going to happen.