Hi,
I think I may have a couple of ideas.
In Sweden the Sandwich cake is very very popular for birthdays. And yes it is sandwiches as a cake.
You get lots of bread (pending on numbers) probably 4 to 6 loaves (white or brown whatever your DD likes), decide on fillings and you can literally have anything (your DDs favourite sandwich)and you start with a base of bread (cut crusts/edges off to shapr it) and work upwards.
My dad insist on sandwich cake every year as he doesn't like sweet stuff.
www.notquitenigella.com/2007/09/23/smorgastarta-swedish-sandwich-cake/
annesfood.blogspot.com/2007/05/swedish-sandwich-cake-by-lena.html
Or you can just google and see what comes up.
They are easy to decorate and top with anything savoury (shaped cucumber/tomato/carrot) or folded ham. You can let the imagination run riot.
And it is fairly certain that your DDs friends will be impressed as its unusual in the UK plus a tasty serving of sandwiches.
My other idea was a plain carrot cake without any icing or creamcheese icing. You can buy glitter gel (edible) to decorate with in Waitrose/Ocado which wouldn't add any sweetness but look pretty for a girl.
They are by Queen Quality Decorations and are called Glitter Writing Gels (comes in pack of 4 colours). I used them recently for my DDs 1st birthday - one of her friends is allergic to dairy so couldn't have cream cheese icing and I thought the babies were to young for pure sugar icing so used this and it was pretty.
You could also use glitter gels to decorate sandwich cake...
Happy baking