Hi! How are you today?
I'm on an iPad mini, iOS 8.4.1, now with 3.7 Gigs of free space on it and it's not enough for the game (especially the Sketch Book) to run smoothly.
I believe your game needs a little less menu animation and a little more stability, lol.
The game itself isn't so bad, it does crash every once in a while, especially if I switch items from the Sketch Book, but it gets very ticklish within the Sketch Book itself. Whenever I try to make a new character, I have to go little step by little step if I'm to use a few colors, and even then.
I am guilty of having 10+ characters and as many pickaxes and weapons. But isn't that a bit of the point of your "Share Your Drawings" option? So people can have a nice booklet of stuff they can switch to? The game isn't very long, so I believe new characters and items is what's going to add length to its playability.
So I could suggest you remove or make optional some of the menu animations and look at trying to fix the game crashes, if possible. Maybe the game itself is too small (in terms of mb's) for its real needs?
Just my two cents. Your game is awesome and deserves to make history in terms of cuteness and creative options - I wouldn't be surprised if it gets you a couple awards and hits the top-something of best mobile games of all times. Seriously.
Unfortunately, the game crashes can get annoying enough to repel some users and make you miss the boat. I believe it's way worth fixing. ^^