Not 100% on what you're asking. But i can try to answer. You can add a fee fields to add or subtract an amount. You can also add conditional fields.
So you could add something like this.
Event 1:
Have it for Adults and have it cost $10.
Add conditional radio button field: "Do you want to add a Child: YES NO
If they click YES then it can add $5 to the total and more fields can appear such as "Child's First Name, Child's Last Name, Age, etc.." This won't add it as a new registration for the child but will just add additional information into the Adult Registration records. But you could then add the information in manually as an "individual" in the backend under "records".
Not sure if this helps, but just a thought.