I have been carefully reading through all your kind replies, thank you, and the overwhelming consensus seems to be to go for it. Sorry I have not been back to the thread earlier, but had to be earlier out of the door this morning for yoga, so a bit short of time.
I am thinking of waiting to see how DDil gets on this week before offering, so will sleep on it for a few more nights.
DGD would have to be delivered to me; I no longer drive and it is a 15 minute walk to their house and I'm not sure whether I fancy that now that the mornings are getting darker. It is also a 15 minute walk to school from there; about the same distance from where I live too.
I already pick DGD up from school two afternoons a week, give her tea and get her ready for clubs. The elder DGD is at secondary school now, and gets herself to and from school.
I think offering a trial run till Christmas perhaps(?) might be a good idea, but will leave it till Thursday and see what DDil's commute has been like this week, and say perhaps I could be of help, and then leave it for her and DS to discuss and decide. After all, they know DGD best.
Many thanks again for your wise answers.