If they're still figuring all of that out, I would say to not worry about park reservations at this point. While they may eventually fill, the majority of dates will continue to be available for months. And though people often think of summer as a busy season, and it is to a point, it is less busy/in demand than many dates in the Halloween season, Christmas season, or random holiday weekends.
Once they know, they should lock in their reservations just to get their plans locked in, but if they need not feel like they need to make their decisions immediately or risk being shut out. There's still a lot of time for them to figure their plans out.
RE: tickets with dates attached-I have never bought tickets through Costco, so I can't 100% confirm that this is true of their tickets, but there have been a number of authorized ticket sellers that have asked for dates when I purchased tickets. This does not necessarily mean that the tickets will be locked in only to be used on certain dates though-sometimes it's just because they want to make sure your ticket order is able to be fully processed before you visit the park. If that is the case, the dates you enter don't actually matter as long as they're not after you intend to start your trip. Definitely read the fine print, but there's a fair chance that it's asking for visit dates just to make sure you'll get them in time, rather than to lock anyone in to specific dates.