The following apps allow you to record and create your own mini-lessons for any topic. You can add voice, images, pictures. There are multiple videos on Youtube showing you how to use these apps. These can be used for teachers teaching/explaining topics, for children explaining concepts, for children to record themselves and a host of options across the curriculum.
2. Educreate Read this article on educreate
3. Explain Everything
If you would like to compare these apps, here is an interesting article.