Great idea to let your niece help with the layout and design. All dependent on her age, perhaps she could get involved in the applique?
A thought re the square sizes ... consider what size you want your finished quilt to be, so it will work out when it comes to assembly, and borders. Also, how it would lay on the bed. A 30" block, would likely mean you would have the joining seam down the centre of the bed. Optically, it would likely be less noticeable, if the seam were not centred on the bed. 30" would leave a lot of leftover material, from the standard WOF, unless you could get a wideback in your colour choice. 1/2 WOF while smaller, would still give you a large space to work with, yet maximize the fabric usage of standard WOF. And in the end three squares would give you about the same width as two of the 30". Please don't see this as me telling you how to do or not do it ... just some food for thought to help you think thru before you get too far into the project. (and yes, I might even end up at the 30" if it were myself though would have wanted to have considered all the possible pros/cons before cutting and sewing)
Good Luck ... will be fun to see this one evolve.