For simple interps I would look at older textbooks, for text and illustrations. Ladybird books are often great sources for pictoral interpretations.
Our Island Story by Marshal (which you can get really cheap) will have some great passages about Chas I.
You might also take a look at 'Cromwell' - the film, as it can be a great resource for classroom discussion on interps.
Christine Counsel's thoughtful activity 'Who cares about Chas. I'. used an extract from C.V. Wedgwood on Chas's execution.
Also read Jane Card (couple of things in Teaching History) and her book on using images for interpretations work