I have been involved in such cases, and am glad that you are finding the strength to do this. It is not only important for you, but also for your grandchildren in the future.
You need to begin with the local authority that has the care of your grandchildren. They will have a contact number - there will be a social worker who has responsibility for them.
I am assuming that you cannot go through your daughter, as that would be the obvious route.
If I can offer a word of advice, I would begin now to write some letters, possibly scan some photos, even make an audio recording, to send to your grandchildren.
In principle, yes, the Children's Services department of the local authority will want to promote this precious visit. In practice, in can be awkward, depending on where they are in care, and who has to spend time enabling this.
Beginning your message to them now gives you something positive to do, and if by sad chance, the visit is badly delayed, your grandchildren will have something to hold on to. If absolutely necessary, the Children's Services department will keep it for them - I have known this to happen - and do not doubt how much it will mean to them.
I wish you all the best with this, and the journey that you are taking.