Has she outright asked you to babysit? If not, don't volunteer!
Affordable day care is something that every family faces, and it's not easy. But one assumes that she and her husband have considered and discussed it before and since she got pregnant!
So just nod when she talks about it, and say "It's so hard, I know, but you and Joe will figure it out." Don't volunteer anything more than that. Just be understanding, like a friend, and not the problem-solver.
Please don't say you "don't work" - yes you do. You have 2 kids at home, a husband, a house, etc. You work all the time.
Moreover, a sure way to ruin a relationship with an adult child is to take over the childrearing of their baby!
When my stepdaughter had a daycare problem while her daughter was in preschool half days, I took day a week. But even that was for an unforeseen problem that just came up with the day care provider. Different family members took different days. A friend of mine with 2 grandchildren who live near her also takes 1 day a week. Once in a while she takes an extra day if her daughter travels for work, or a weekend once in a blue moon. No more than that.
You are not selfish for not providing 40 hours of daycare. By that argument, your daughter and her husband are selfish for having a baby without any plan to care for and parent it. This is their child, their need to find acceptable solutions, their job to find family leave from their employers, and their job to run the numbers. Don't solve their problem for them - let them research it, discuss it, and come up with a solution.
And no one can make you feel guilty without your permission.