You don't say if your family for purposes of the trip involves just your parents or if you have a spouse/children or anyone else going.
If it's just you and your parents going, all you can really do is keep stressing that you will be there at one of the busiest times of the year, meaning lines and crowds will be everywhere. Maybe you could find some historical attendance figures for that time of year and throw some big numbers around. Maybe hearing that x-thousand people could be there that day will be a wake up call.
If you have spouse/children and your parents are coming along too, employ above strategy, and....make sure your IMMEDIATE family is taken care of. I'm assuming you already have your hotel booked. Make a couple of key meal reservations, including your parents in the count. Be prepared to employ whatever park strategies you feel need to be done.
If your parents wait too long and can't find lodging, I know everyone will be really disappointed, but....it is not YOUR problem; you tried to tell them! Assuming they can get a room, when mealtime rolls around and everyone is hungry, you will already have ressies. If they want to join you, great; if not, you simply go without them and inform the host that your party size is less than expected. If you want to be there at park opening and they don't, you just GO and agree to meet up later, or have them call/text you when they arrive. It's okay to tour separately at times.
Traveling with extended family can be hard. Plan for your immediate family's needs as best you can, and be willing to bend a little so that your parents don't feel they are having every minute dictated to them. Get their input on some decisions once you are there. The one thing you don't want to do is say, "I told you so!" if their lack of planning backfires on them.
Good luck with this. Last year I decided against inviting my mom and stepfather on our trips to avoid these kinds of hassles. I'm a detailed planner, but my mom is an over-the-top super planner, and we would clash horribly!
