AIBU to feel put out by my daughter's changed school run request?

Good on your daughter having firm boundaries. It’s easy to read through the lines and see there’s way more to this than you’re claiming (but then I also grew up with a mother who was competitive with the in-laws, preferred my sibling to me, and criticised my parenting. We are no contact now).

OP, your daughter does not like her brother.
This is clear.
They don't like each other.
Yet you feel it appropriate to be filling her in.
She doesn't want to hear it.

Also your passive aggressive remarks speaking through the children that "mummy is taking too long".....would piss anyone off.

Instead of helping her out, you are stressing her out.
She has now made other arrangements.

You can be ourtraged and righteous or you can seriously reflect.

If I were you I would stop with the texts, stop talking about her brother, and stop with the PA remarks.

You are getting on her tits and she is backing away from you.

Strikes me that the dd is doing her best to try and maintain a relationship with her mother, and facilitate her seeing the dgc very often throughout the week despite the OP being really annoying.

Keep in mind, that is the impression the OP has given about herself, on a thread which she started obviously hoping we would all agree with her.

This is even without any of us having heard the situation described from the point of view of the OP's dd.

It sounds like your issue is that DD enjoys looking after her own children, but you want to do it instead.

You have had your time being a parent, let DD parent her own children.

She is asking very little of you. Of course you should do what she's asked. Why would you think its a good idea to insert yourself in the family morning routine.

And she's welcome to take her own kids to school as well

But her convenience is the entire point of the arrangement, isnt it! It’s to help a busy mum at a particular time in her day when she cannot bilocate.
A family social element, as in grandma + kiddies time, is not the point at all.
(Unless that virtual quid pro quo were actually discussed when this help was being arranged. She’s helping her daughter, but ‘recompense’ shouldn’t be assumed, unilaterally.)

The mum may want every available moment with her children before they go to school, as they are very young, and she surely misses them all day.
I don't think it odd at all that she has made this request to her own mother, but it’s a shame she needed to.

It'd be a much bigger shame if OP’s umbrage were to become a concern between them.
At best, both women have the higher ground here, and in order that their easy relationship isn’t damaged, I think it has to be OP who exhibits graciousness, and gives way despite her own wishes, and without demur.
Ultimately, the children’s mum’s need for that last half hour with them before school/work trumps her own mother’s wish to spend that bit of time there too.

(I realise my generation may not have seen so much of our grandparents as modern kids do though. Even though we lived in the same little town when we were primary school kids, we saw the old folk less than weekly, never mind thrice weekly!)