This is a really tough one because so much of it depends on the personalities of the people involved, especially in such an emotionally charged situation. What might be touching to one family might be depressing or too personal for another; one person might want to laugh and show that darn it, they are going to live every moment to the end with a personalized wine glass while another might find that insensitive, and so on - so hard to say without knowing their temperament.
A few ideas that fall more on the 'maybe too sad or sentimental for some, maybe right for others' side:
If you search on Google, you can buy an "Our Adventure Book" scrapbook in the style of Up, which you could fill with pics at Disney over the years and memorabilia / favorite quotes and memories from the past. (That may hit too close to home as they look at the book in the hospital when Ellie is terminally ill, so if not that, maybe a different Disney scrapbook.)
A charm bracelet with different characters, with a DIY book (you can find an image of a character on Google images and past it into a word document with your text, then cut and print each page, hole punch at a corner and tie it together with ribbon) with a description on each page of how she resembles a certain character (shares their strength, bravery, sense of humor, and so on.) Maybe with a focus on the traits she will be drawing on towards the end, with the bracelet as a reminder.
A personalized Disney autograph frame (you can find them on Etsy) with all of your signatures and a meaningful pic.
A personalized framed quote (something you could DIY or order from a place like Etsy) with a Disney quote sort of appropriate to the situation (looking on Etsy, many of these quotes are already available as prints, pendants, and charms.)
You may be gone from my sight... but you are never gone from my heart - Winnie the Pooh
I will ride I will fly chase the wind and touch the sky - Merida
There is nowhere you could go that I won't be with you - Gramma Tala