I haven't read all the replies but a few things struck me here. You have sent this just before Christmas which is a very busy time for most where their head is all over the place. Was this the best time? Have you offered it as some sort of Christmas present, in which case it seems more like a loaded present to answer your needs than to answer theirs at this time. You also want them to act grateful when actually it seems a bit like a present with pressure attached (they should be grateful, you are paying therefore it's for them).
I don't mean that to sound harsh as it is not unfair for you to want to spend time but it seems a little manipulative to me to do it in this way. It makes it awkward for them to refuse both because it's you paying and because of the timing. This might be why they haven't replied straight away and it is a bit unfair to be upset with them not immediately responding or being grateful.
You paying is kind, but could add pressure in that it might make people feel like they would have to be grateful throughout the trip and have no say over what they do, or if they wanted time out or do things apart for some of the time.
It strikes me that your granddaughter is still extremely young and your DIL and son might be at that very anxious and early stage. This might be hard to communicate and they might be a bit more relaxed in time. I would take the pressure off them for weekends away which might be too long for them and out of their comfort zone. If I were offering a present I would try to think of a present purely for them, not trying to use it to get more time or whatever you are wanting out of it. Maybe ask them what they really want. You could get them a weekend away without yourself and husband going. Don't use presents for other motives, it just adds guilt and awkwardness.
In terms of seeing more of them, that is really a separate issue but I'd start smaller like an afternoon trip out together or something like that or a meal out, rather than pushing for weekends away where people might be irrationally stressed with a young baby. If they are reluctant, it might be more to do with anxiety or early parenthood more than to do with you so it's worth bearing that in mind and trying not to take things personally. I would try not to let resentment build up during this early time but remember people can get more relaxed when their kids getting a little older so things can change a lot.