We have not left our DC for as long as 12 days, I think the longest was 5 days. They are now 4 and 6yo. This was about a year ago so they were 3 and 5yo.
I would let them dictate how often they call you. I found that they often did not want to speak to me anyway (they were having so much fun with Granny) and that it unsettled them.
They won't forget you, but children of that age have a limited understanding of time. They don't know if it was 10 mins ago or three days that they last spoke to you.
I leave a list of emergency contact numbers when we go away. Also numbers of friends in the area so that they can ask for assistance if need be.
I don't bother with lists of food/activities any more, as they are used to Granny and Grandad and will tell them what they like. I do make sure that I fill the freezer (and sometimes make a lasagne, or shepherds pie so that my parents don't always have to cook).
Does your mum like to bake? My DC love baking and it is great for a rainy afternoon. Make sure you have ingredients in the house.
I try to make sure that I have everything in the house that they might need (food, loo roll, drinks etc) so that Granny does not need to do a lot of shopping.
For my Mum I try to have a bottle of Baileys in the house too so that she can have a wee glass in the evening when the kids are in bed