The chances of your daughter being unaware of the situation are slim to none. What do you think you could actually do to help, as opposed to distancing yourself? You can’t really distance yourself unless you are already close to the situation. Did you teach her to cook healthy meals when she lived with you? Has she asked you to get involved?
I think the last thing people want is unsolicited advice - particularly when it comes to parenting, as it tends to come across as criticism, and comments about weight are extremely personal.
How much do you see of your grandchildren? If you are cooking for them you could try to introduce healthier foods, but many children will prefer to eat familiar foods, so it might not be easy.
I would say nothing unless she asks for advice, and as others have said, if you are looking after the children maybe do more activities with them.