Expecting a grandparent to take charge of two young children, and having to get up at 4am to do so, is a very big ask. Does your DiL also have a very early start for work? If so, I can understand how difficult it must be for them to arrange childcare at that time, as Nurseries wouldn't even be open. But, if she starts work at a more reasonable time, I fail to see why you were having to take charge so early. Were they dropped off at your place, or did you have a long journey to get to them, hence the early start? Either way, I think you're a saint for having agreed to it in the first place.
You say that there are no issues on your DiLs side of the family ... I'm just wondering where her mother comes into all of this? What childcare support does she already offer?
It's very sad that after all you have already done for them, they expect more, and because you are unable to offer it, they have excluded you from their lives. How selfish of your son and DiL not think about the impact that taking care of 2 young children would have on your life, and for such long hours.
Did you have a good relationship with them beforehand? ... I assume so, otherwise they wouldn't have been asking you to help with childcare. That being the case, after 3 months of not seeing them, or your grandchildren, I would have to approach them, and try to sort this out with them. Whilst you're only 60, I can totally understand how you feel ... after helping out with my own grandchild for a while (I'm 58), I felt so exhausted ... it's definitely a young person's game! Tell them how upset you are that you no longer see them, and you never thought it would come to this. You tried to help them out with childcare as much as you were able to, but you need to explain again how difficult/exhausting you were already finding it, and you couldn't have coped with extra days (even though you'd also offered to take them on a Friday).
I do hope that they can see reason, and realize that you have not done anything wrong. What a terrible shame for your grandchildren to be estranged from their grandparents because of this. I wish you luck in sorting this out.