So, you never go out in the afternoons and your mum shouldn't have done too?
I get why you are a bit miffed, you got in, you thought there'd be upstretched arms, everyone shouting 'mummy, mummy' and basically they've all gone out.
But when you ask other people to look after your children, they don't do the same stuff, feed them the same etc. If it is a toddler, it's not a tiny baby, surely even the strictest routine has a little give in it?
I would think having a night off, plus getting the children to see your gran is a double-bonus. The children will be fine, you are fine, there's nothing to fret about, honest!
And- perhaps your mum will come back and think she didn't enjoy sitting on the M25 and make a mental note not to do that again with small children, but I don't think she's done anything wrong by taking the children to see their greatgranny. Mine does a lot of outings and family visits with my children, saves me doing it.
The most unreasonable one is your husband!