Thanks for all the feedback guys!
App like this should never replace real thing! My wife is children book illustrator (and created the first set with animals) and she draws with our son on paper a lot.
This app can help you in certains situations, where you need to entertain your kid for a while and phone is the thing you always have.
I am also getting mixed feedback about buy buttons, so I will experiment tonight and will bring better solution which will be even less annoying. If you have good examples or suggestions, I'll be happy to see them!
Thanks again.