So here is my very first attempt at any kind of cake, baked last week... certainly an adventure. Seemed to be received ok cake loving wife...
I have these deep tins that I bought for dishes other than baking. One is 8 inches, the smaller one is 6 inches (just under 3 inches deep). I used the 6 inch one for making a chicken fajita stack, looks like its in for a new life with cakes. I haven't used them for a while, I was looking at deep tins online when I realised I've got two somewhere...Your best bet will be a 6 or 8 inch, and yes, a single layer will be fine. I usually find a 6 inch is a nice size, and then I place it on an 8-inch cake board. I use a deep tin, though and then level fill and cover it. The reason being is that I get straight edges and no ridges when covering it. When you sandwich two together, you can sometimes be left with a ridge around the edge. Anyway, have a look around the net, get some ideas from your granddaughter and then we can look at some options for you.
They are both loose-bottomed, I'll see how I get on with the 6-inch one as is. I can always buy a solid one.Is the 8-inch loose-bottomed? I think from the looks of your picture; the 6-inch is solid. I prefer these as that way; all the cake mix stays where it's supposed to, and you get a perfect edge when you flip the cake over. (When you level fill and cover a deep cake, the bottom of the cake becomes the top because it's smoother and easier to cover, giving you a smooth edge when covering in icing/fondant). However, you do what suits you; what works for me may not work for you, so choose what suits you. When I level a cake, I always use a cake leveller too, so it comes out straight.