As others have said, I wouldn't make up their mind now for them.
I lost my son one July, and it truly baffled me that life goes on. You feel that you will have to curl up and die with the pain of it, but sooner or later you find something to laugh about. I remember being excited about picking a costume out for my then 2 year old daughter for Halloween 3 months after he passed away, and feeling a bit of guilt for looking forward to something, but as many others have said, looking forward to things and realizing that we can go on after a cherished love one is gone is a beginning to the healing process.
These girls could very well be holding on to this trip as the one tiny bright thing on the horizon. Children grieve very differently than adults. While the thought of doing something like this might exhaust and overwhelm some of us as adults in this situation, children still need to play and be, well, children. I cannot say what this poor woman is going through right now, but I know that she probably has great concern for her children and would love to find a way to help them through this process. Now might not be a good time to broach the subject, but odds are that she would be probably be relieved and touched that someone would care enough to reach out to her kids at a time like this.
My thoughts and prayers go out to everyone in this situation. I hope that the trip works out and it can bring a measure of happiness for the family.