Writing your own content works exactly like it does for any other account - see Writing your first topic and From draft to published for the describe-generate-review flow, which is identical here. The one difference: because your site has age groups instead of classes, you'll be asked which group this topic is for when you describe it.
Tagging a topic
Pick one of your site's age groups, and the finished topic only appears in that group's session - a child in a different group never sees it.
Leave it untagged (or don't pick a group at all, if you're writing something that suits everyone) and it appears for every group at your site.
Nothing else to set up
There's no roster to assign a topic to and no per-child visibility to manage - tagging a topic by group, or not tagging it at all, is the entire targeting decision. Once it's live, it simply appears the next time a child in that group taps in.