I used iBooks Author to create an iBook. It is designed to help children understand what length is, and to help them to build a collection of useful referents for centimetres - but it will work best if there is an adult beside the child encouraging, questioning and supporting them through the exploration.
You may download it to your iPad (that is the only place you can use it right now) by clicking on the link below. It is about 14 MB in size, so it may take a couple of minutes. When it is finished downloading - open it in iBooks.
Share it with a child and share it with your friends!
Send me feedback (firstname.lastname@example.org)