A little too simple
The buttons are much too small to touch with big fingers like mine, but I understand that making them bigger would result in less canvas space. To that end, I think it would be smart to add tools to adjust the canvas size and to zoom in and out of the canvas.
The drawing tools are as basic as can be. You have a pen with adjustable size, a preset palette of colors, but surprisingly you also get a line tool and a square tool. If they were going the extra mile for a square tool, they should have also gone for a circle tool at least.
This could be a really good app if it only had more tools and features. It is a little too limiting as it is now, but many basic or casual artists may not mind for the novelty of drawing together.