Here are some questions I would like to suggest that you ask yourself before making up your mind:
Do you enjoy your work?
Can you afford to retire now?
If you retire and spend 1 day a week looking after your grandson, what are you going to do for the rest of the week?
Do you enjoy driving?
Who will be paying the petrol? You, or your daughter?
Why does she only need child-care one day a week?
How well do you and she get on?
Will she be paying you, or are you helping for free?
Do you enjoy looking after small children?
I love being retired, nothing would persuade me to go back to work, and I love looking after children, but that said, I don't think I would drive either 2x 35 miles or 2x 70 miles to do so.
But it all depends a great deal on how you and your daughter get on.
There are lots of posts on here about the joys and problems of grandparents providing child-care. Have you read them? If not, please have a look.
It is hard to say no to our children, or other family, and even harder to imagine that problems might occur.
If you decide to retire now and agree to your daughter's suggestion, please sit down with her before you do anything else and work out EXACTLY what services you are expected to provide, discuss discipline, food, shopping etc. and write down what you agree on. It may come in very useful at some point in the future. And remember to discuss whether she can expect you to jump in at the drop of a hat and stay for a whole week every time the little one is ill, as I assume she cannot stay off work to cope with a sick kiddy.
I would also ask her how long she envisages this arrangement going on for. Do you want to be tied to one day a week at her place until he starts school?
I would also tell her that driving to hers, looking after a toddler all day and driving home again afterwards is too much, so you will need to spend the night at her place, either, preferably, as you will be tired after looking after the little one all dayat the end of that day - but that may not be practicable if it means a very early start for you in the morning. So driving to her the day before, staying overnight, looking after your grandson, then driving home may be the better option.