Thanks everyone!

Nanna has seen a nutritionist and he suggested increasing the fat content. Like eating her cereal with pure cream rather than milk. So I don't think I have to be too careful about offering low-fat food. If anything, the opposite.

I was also thinking of things like salmon and vege (maybe peas) quiche. Eggs are packed full of nutrition, too.

Oh, and by the way, I'm not their granddaughter, they are DH's grandparents. So that makes it a little harder as they love me, but don't want to admit to me that they need help. Dh's mum died of cancer when he was 9, they do have another daughter but she lives in Sydney, and TBH, she's not responsible enough to look after her parents. She's in complete denial about their health and age. Anyway, so it's DH and I who just live around the corner from them and we really do want to help as much as possible.
DH's dad is close to them, too, so is his step-mum, but they feel it is not really their place to step in too much. They do a fair bit already. The grandparents pretty much raised DH when his mum got sick until his dad remarried. So Dh is very close to them. They also have been very lovely to me, kind of taking me on as another grandchild (my grandparents have all passed and all my family is in Germany), so I love them to bits.
They are too proud to access other methods of help like meals on wheels or household help, which makes it really hard. If you ask me, they are in no state to do things like changing the sheets or doing the laundry. Until a couple of months ago they were looking after Nanna's 95 year old sister. We pretty much forced them to put her in a home. It was killing them looking after her.

Anyway, this is kinda off topic.
Thanks for all your input.
Saša