A fave: iPiccy has many easy to use photo tools, enabling you to edit pictures, apply photo effects, add text or paint freehand from scratch.
iPiccy is free and no registration is required.
The iPiccy editor seems accessible enough that children, of many ages and abilities, can edit and create artistic reworkings of photographs, without too much of a learning curve.
As well as retouching individual pictures, it takes very little time to make collages of multiple images. (Requires temporary access to a local drive) but SO handy.
Category: 2) Useful n Interesting