Teaching Buddhist concepts to a child
Try the videos under the section of Children on this website.

There are some ebooks for children here.

There's a simplified introduction to Buddhism for kids here.

There are websites on Jataka Tales for children here, here and here.

I think a great way of teaching Buddhist concepts to children is through stories. Stories are entertaining to young children who need their creativity and imagination stimulated, in order to learn things happily and efficiently.

Instead of trying to teach very young children concepts like anatta which may be too difficult for them, it might be better to teach them virtues and good moral values, which are available through the Jataka Tales and some Dhammapada stories. This is the foundation to inculcating sila.