You can tap on the export icon on the top right toolbar of Sketch, Photo and Doodle Desk, to invoke the Export panel.  
While on Coloring Desk, the export panel will open up once you click on the check icon on the top right corner of the screen and on Kids Desk the export icon sits on the right side toolbar.  The export panel gives you multiple options to share and export your artwork. Save to device - Save your artwork into your device gallery. Export as PSD - Exports your artwork as a Photoshop-ready PSD on Sketch and Doodle Desk. Quick export - An easier and faster way to export your artwork among a range of destinations and apps.