Hey there, Jordan!
Welcome to planDisney, and thanks for bringing us your question!
It's great to hear you and your family are planning a visit to
Disneyland Resort. Enjoying all the magic is even more fun when the whole family plays together.
One helpful note to keep in mind about reservations you make through the
Theme Park Reservations system is that reservations can be created and canceled, but cannot be modified.
What this means in your case is that there isn't a way for the Guest who made the theme park reservation to modify the reservation and add you to the reservation after the reservation is made. Guests can cancel a reservation and make a new one based on reservation availability.
It's also helpful to note that the difference between starting in one theme park or the other is a difference in the theme park reservation rather than a difference in the theme park ticket itself.
My best suggestion is to
keep checking the Theme Park Reservation Availability calendar for the date you wish to visit Disneyland Park, as it's possible that park reservations might be made available on a rolling basis.
Jordan, here's hoping you and your family get to enjoy all the magic of Disneyland Park together!
Jerry