Welcome to disABILITIES.
If someone is using a wheelchair, they generally won't need a GAC (Guest Assistance Card) because seeing the wheelchair tells the CM (Cast Member) that they need an accessible entrance (which is basically what the disabled entrances are). Many rides/lines are fully wheelchair accessible and there is not a separate "disabled entrance". If there are additional invisible needs besides mobility needs, the person might need a GAC. The GAC is not meant to (and often won't) shorten or eliminate the wait in line. It is a tool to quickly let the CMs know what sorts of assistance are needed by someone with a disability.
The GACs are not given out based on diagnosis or condition, but are based on the needs of the person with a disability. Just like the rest of us, everyone with Downs Syndrome is different. Some have little difficulty waiting in lines and don't need any special accomidation, some have specific things that are a problem (like maybe climbing steps or exposure to direct sun), and some have more difficulty and might need a quiet place to wait, away from a lot of other people. Some might have vision or hearing disabilities that make sitting in a specific area important. Because everyone has different needs, there is no automatic GAC for Downs Syndrome. In order to get a GAC that meets your childs needs, you'll need to go to Guest Services in one of the parks with your child and explain what sorts of difficulties you expect/what your needs are. That may sound difficult, but it really isn't. You just need to tell the CM a bit about what sorts of situations cause problems for your child so that they can help provide the type of assistance that will be helpful to you.
The GAC is usually issued for up to 6 people (5 plus the person with a disability). There might be times when you will be asked to split into smaller groups (sometimes it's because the alternate waiting or seating area won't accomidate 6 people or the area is already quite full/expected to be full). Gacs given out in one park are valid at the other parks and the GAC will usually dated to expire the day your vacation/trip ends.
If you are going during a non-peak time, the park may not be very busy and you may find that you don't need a GAC at all. When the stand-by line is 15 minutes or less, you should just use the stand-by line if possible.
If you have other questions, or this didn't answer your questions, feel free to ask more.