To add:
I extended my parents stating that they will attend the 1st birthday of my child. Maybe, you can state continue spending time with parents or something similar. I don't think it matters that much as long as it is valid and reasonable and it's the 1st extension.
In my case, I submitted the old return tickets, new changed tickets, my kiddo birth certificate.
You should submit all documents that supports your case. The more the better as long as it makes sense and supports your reasoning for extension.
Also, don't file too early. I filed right around 35-40 days earlier before expiration of their I-94. Because USCIS takes longer time to review cases like this, it works in your benefits not to file too early.. And as long as you have receipt notice (this will not take long, you should get a receipt notice in 1-2 weeks), your parents will be legal status until decision is made. So even if you receive extension denial, your parents had already spend months by the time the decision reaches to you. And also, there is no mention of days that you should return after denial. Effort should be made to return as early as possible in case of denial so that future visits are not affected.
In my case, My parents returned after 3 months and I still didn't had extension results from USCIS so I wrote a letter back stating to pull application. I received confirmation in mail.
Also, I am sure you know it, But you just need one application for all family members as long as they are related. This saves you money $$$$