It's a 'Guess the Weight of the Cake' thing, and one of the mums has been on a cake decorating course so is keen to decorate it. I'd like to do something Autumnal like an apple cake but that doesn't really lend itself to decorating does it?
Only other thing I can think of is a boring old fruit cake and let her do the autumnal theme in the decorating.
It's sort of a 'combination' of the two cakes you've suggested, in that it's a fresh fruit cake, as opposed to a dried fruit cake, and it includes apple. To be clear, I substitute half of the carrot with the same amount of apple (cooking apple or granny smith, for example). I leave out the nuts as I'm not big on nuts in baking, but you can leave them in.
Use the size of grater that you would for grating Parmesan cheese, as opposed to regular cheese. I found that the regular sized grater made the carrot pieces too big, although the regular grater is fine for the apple. Slightly more work for the carrots, but totally worth it.
This cake has become a family favourite, plus I made it once for DH to take into work one day and apparently it was demolished in minutes. It's so easy, as well.
It lends itself perfectly to a cream-cheese icing - if you made a relatively firm icing (extra icing sugar, not too much cream cheese or butter) it should work well for decorating.
Just to add, mine seems to take a bit longer to cook than the recipe suggests - just keep testing it with a skewer. It will never come out 100% clean as it's such a delicious moist cake, but you should still be able to tell when it's cooked.