Currently Simple.Data does not support saving object graphs in one call so you need to Insert the parent, save it's ID, set it on all children objects and finally Insert all the children.
This might improve in Simple.Data v2, as you can read in this post by Mark Rendle, the author of Simple.Data (section Better WithStuff): http://blog.markrendle.net/2013/09/18/simple-data-2-0/ .