OK.
This is a little complex, and the best (for your ease) would obviously be to wait for the new M2M Relationship features coming in Types, but I cannot expect you to wait, because well, it will still take a lot of time.
So let's set this up the classic way.
I don't want to create (new) Events for every Rodeo I create. I want to re-use the Events or even have them available to more than one active Rodeo.
After creating the Rodeo and Event CPT, create an intermediary Post type. Make it child to both the Rodeo and events.
This Post type will allow you to connect many Rodeos with many Events.
It's elaborated here:
https://toolset.com/documentation/user-guides/many-to-many-post-relationship/
Your main post type that you will query is the Intermediate post type.
From that, in a View, you can call the parent Rodeos and parent events.
The events are gonna be categorized with a Taxonomy (age range).
I think, your intermediate Post Type can be the "Childs".
So when a User creates/adds a child, he will choose:
Rodeo Parent, Event Parent, and age.
If he needs to register the child for a new event/rodeo, a NEW Child is required.
With the upcoming Many To Many relationships updates in Types, this will be much easier.
But that is not yet something I can show you, hence, if you are in a hurry, I suggest to go with the classic methods.