Lol! The toppers will be great fun and give you an idea of how to work with icing.
That looks nice, the buttercream looks good, and the cake looks moist even though you had temperature issues.
The thing with cake is that you can always cover cracking so that wouldn't be a problem. If it's burned or dried out, you have a problem.
I like the fact that you're planning ahead and doing a test run; you'd be surprised how many people don't.
Maltesers are the easy part; crush them up and then sprinkle them in the buttercream centre, so that's that bit taken care of.
As for the cake, I would need to see a picture of precisely what you're looking to do to advise you best; some of the unicorn cakes are covered in buttercream with a unicorn horn and piping on the top, some are rainbow coloured inside, some are covered in icing and have a rainbow and unicorn on the front or down the sides. So with so many to choose from, I could be advising you for hours.
What size are the cakes you've just made? They look about 8/9 inches on the top pic but then on the close-up, they look to be about 7 inches. So I can't gauge it.
If you're doing the buttercream and horn version, you're going to need some more height to the cake and may possibly need three layers of cake to get the height, but if you were looking to do that, you would need about a 6-inch cake to make it look in proportion.
The bigger the cake, the more problems you can have with it, so it may be worth making your granddaughter a smaller version to start with. Start smaller, then work up to bigger cakes.
Anyway, have a nosey on the internet, see what you like on the unicorn front and then post a pic that way; I can physically see what you want to make and advise you accordingly.