I created a new object (e.g. a discount, a new custom page, a new suggested message, a milestone, a new participant type, etc.) in my blueprint. Then I tried to push that down to the existing children events which didn't seem to work.
Once a child event is created, new objects created in the parent will not push down. If you anticipate needing additional objects at the time of creating the blueprint, you may want to pre-create some additional objects so that you can modify them for future use as needed.
The exception to this is survey questions, which can be added to a blueprint and pushed down to child events even if it was not created before the child event was created.