As arubalisa mentioned, what you are aking about is a Guest Assistance Card (GAC). It's not a pass since it doesn't give immediate access and it's not meant to shorten or eliminate your wait in lines. It is a tool to let the CMs know what sorts of assistance/accomidation someone with a disability needs.
Because each person's needs are different, the card for one person may not necessarily have the same messages as the card for someone else. You can go to Guest Relations in any of the parks with your children and explain their needs to the CM (what you said in your original post is fine, although you might want to explain a little more about the difficulty you son would have waiting in line - don't do it on the boards though, we don't want to have a lot of specific information). It's actually better not to know what the messages say ahead of time - the CM might actually suggest something you hadn't thought of. (Although, I will tell you that there is a stamp to allow a stroller to be used as a wheelchair - but, if you said to the CM what you posted, you would get that one even without knowing it exists).
If you plan on all staying together as a group, you probably only need a GAC for your youngest child. The GAC is usually issued for up to 6 people (5 plus the person with a disability) and the GAC issued in one park is valid at all the theme parks (but you will need to talk to Guest Services at the water parks if you go to one of them). There may be times when you are asked to split into smaller groups; usually that's because the waiting/seating area for special needs guests for that attraction is small or is alreay crowded.
Other things that will help are getting to the park as early as possible in the morning and using Early Entry if you are staying on site. By doing that, you can often get on rides with no or just a short wait. If you go in the morning, leave by about 11 am and then maybe come back in the late afternoon, you will miss the busiest and also hottest times to be in the parks.
Pixiedust Inn (one of the links the arubalisa posted) has a lot of good info.
