do we order tickets in advance? If so how do we do this for two seniors, one adult and one child

You pay at the window before going into the park

Unfortunately they do not sell tickets on line. You just pay when you get there. If you are a member of the Ohmaha zoo it gets you discounts there. They are part of the Henry Dorley zoo

What they said.

You pay when you ready to go and drive into the park