YABVU
You would not unreasonable had you simply decided that the arrangement suggested doesn’t suit you- I can see it would potentially be quite tying to provide care for such a short period 3 days per week. But that is not the reason why you don’t want to do it, is it? The real reason that you don’t want to do it because you aren’t getting exactly what you wanted. It’s childish and manipulative, if you put it like that to your daughter.
From what you have written in subsequent posts, I can entirely see why your daughter would not want you to do the level of care you would prefer. She has legitimate reasons, which you simply dismiss. If you dismiss her decisions in this matter because you don’t happen to agree with them, what else might you decide to dismiss or ignore, should you not agree with her choices? You can argue all you like that you’d respect her wishes when it comes to your granddaughter, if we’re caring for her- but you aren’t demonstrating your willingness to do that thus far, so I’m not convinced. Perhaps that’s true for your daughter too?
You also sound quite callous when it comes to the fact that your grandson looks to you more than his mother at times- you can’t see that this would seriously upset some (? most) parents? Moreover, you seem to actively WANT that- I’d be quite aware of how it could be upsetting to your daughter/her partner and in your shoes I’d try to encourage the child to look for mummy/daddy too. You seem more than happy about it though, which doesn’t come across well. It reads a bit like you are in competition with the child’s mother/parents to be the favourite/the one they look to for comfort. That is a huge issue and, if accurate (as opposed to just the way you have written it), something I suspect your daughter has seen this play out with your grandson and has thought better of the arrangement. Perhaps she also thinks you’ve overstepped other boundaries with your grandson, or is worried that you will with her daughter- given your dismissive attitude towards her choices?
Also, she perhaps needs the certainty- if you are sick/suddenly have to change work days (temporarily or permanently), have an emergency elsewhere, an appointment you can’t change etc, she has no flexibility with nursery to add on hours. This way she knows what she has to cover (bar her DD being too sick to go to nursery) and can make suitable arrangements. You don’t have to like her choices, you do have to respect them.
Having grandparents do childcare can be wonderful, but it can cause issues too, even if both parties enter into it with the best of intentions and good relationships. In my own circle of friends, I have seen grandparents use the childcare they provide as leverage to manipulate their children to do things as they wanted/when they wanted, grandparents who thought providing childcare gave them the right to interfere in the way their grandchildren were brought up to a staggering degree, and resentment on both sides (e.g. disagreement about something small, parents want things done as they prefer, grandparents feeling under appreciated/ distrusted when they are doing their child a big favour by caring for the dgc). It can work well, but does need careful planning, boundaries on both sides, and a lot of mutual respect.
You don’t have to like her choices, you do have to respect both her right to make decisions about her daughter and the decisions she makes (unless she is abusive or neglectful, obviously). Especially since you have asked her about why she has made this choice and she’s explained her reasoning. It sounds like you have expressed your doubts about her reasons from what you’ve written (perhaps not), and despite that she still plans on using the nursery. Now you have to leave it there- you’ve asked her why, she’s told you and you’ve had a chance to state your views for her to consider. Despite this she’s chosen to go with her original plan. So, if you want to be a good parent, you accept it with good grace and decide whether you would like to do the nursery pick ups to spend time with your DGC or not. If you don’t want to, feel it would be too much of an inconvenience for a short time with her or whatever, then that’s absolutely legitimate and fair- just explain to your DD that isn’t agreeable to you. She would turn be unreasonable if she did not accept your decision with good grace.