Open to suggestions on how to manage 4 kids (ages 18 mos, 3,6 & 8). What are the best park passes to buy for a 3 day stay? Where to eat? What you can bring into the parks (food etc.). Best rides. Can you rent strollers? Do I tie my kids on a rope to keep them close? Where do you park your car? Got myself a AAA Parking pass (anyone care to comment on them?) Do I bring Junior (18 mos), or get gramps to stay at home bas while we take the others?
Also concerned about one kid being able to ride, when others have to sit and wait (getting bored or worse - lost).
Well, I'll give you the same advice I gave my brother when he was taking his 3 kids (ages 4, 6 & 7) for the first time.
The first thing to do is to pick up a copy of The Unofficial Guide to Walt Disney World and read the chapter entitled "The Ecstasy and the Agony." It doesn't have to be the latest version, anything from the last few years will do. I'd just grab one from ebay for a few bucks. There's also a great section that describes each ride and what age groups they're intended for.
Also, don't worry about kids having to wait while another in your group rides a ride. Everyone will get to ride together. When you get to the front of the line, the CM (person in charge of loading people onto the ride) will ask you how many you have in your party, and will organize you according to party size. So, that's something you won't need to worry about.
Regarding the stroller.... I know a lot of people buy a stroller, but if you're just going for a couple days, and your budget can handle it, I personally would opt for the rental stroller (double version). Disney rental strollers aren't like regular strollers. They're kind of like a plastic papasan chair on wheels (you know those round scoop chairs they sell at Pier 1). Also, there's probably a photo somewhere on the internet if you want to try to look up what a WDW double stroller looks like.
Anyway, you should be able to fit 3 of the four kids in it at any one time, switching around to take turns, etc. Yes, they'll be kind of squished, especially if your 8 year old climbs in, but the 18 month old will probably be able to sit on a lap. Anyway, it offers all kinds of versatility, so you should be fine with just one of them. If you find you need another, you can always rent another one later in the day.
Another reason I'd go for the rental in the park is that the trip from where you park your car to the actual gate of the Magic Kingdom is not stroller friendly. You're going to have to ride a tram (unless you get a super-close parking spot) and then either the ferry or the monorail, and, in my opinion, having a large stroller to manage through all of that is a pain. If you have 4 adults (you, wife and grands) and 4 kids, you'll have no problem getting through all that by holding hands/carrying the little ones.
About food... you can bring an entire picnic into the park if you wish. Of course, you don't want to lug around a big cooler all day, but a small soft-sided one will fit into a locker just fine. (Just call WDW for locker dimensions, or, again, it's probably listed online somewhere.) At a minimum, I would definitely have a backpack with some juiceboxes and snacks (granola bars, raisins, etc.) to ward off hunger between meals. You might also want to bring in some throw-away ponchos (wal-mart for $1) but only if the weather for the day looks iffy.
Definitely dress them in a bright color that's easy to spot. Orange and red are great. But, I wouldn't worry too much about getting separated. You'll be spending a great deal of time in the ride lines, where it's virtually impossible to misplace a child. Yes, the crowds are big, but in my experience, kids are more likely to stick close to parents when encountering a new situation and a large crowd of people, so the likelihood of someone "disappering" is pretty small. Oh, and if you haven't read up on it yet, read about "fastpass" so that you can use it. It's great!
Regarding whether to bring the 18 month old -- I would, at least on the first day. S/he may not make it for the second day (and that may go for gramps as well!), but kids that age definitely enjoy Magic Kingdom. Plus, then you won't have to hear for the rest of your life about the time everyone went to disney world and s/he had to stay at the hotel.
The main thing you're going to have to force yourself to do is pace yourself. I would plan to get to the park early on the first day (before opening). Kill yourself to do this if you have to. You'll have a much better experience if you don't have to fight the masses and wait in long lines from the get-go. Go to Fantasyland first, because it'll be packed from about 1 hour after opening until about 8 or 9 pm, and that's where most of the rides your kids are going to want to ride are located. Now, around mid-afternoon, you'll be tempted to soldier on, get your "money's worth" and do one or two more rides. Resist this urge. Go home, get naps, enjoy a quiet evening, sleep in on day 2 and go to the park after nap time. This way, you can enjoy the park in the afternoon, ride some rides in the evening and see the evening parade, and if you're really lucky and everybody can hang in there, the fireworks.
Trust me, there is no way you'll be able to go from morning til night one day, and then turn around and do it again the next day. (This advice is kind of like when you tell your kids not to drink hot chocolate through a straw. You know not to do it only because you've done it yourself and been burned. Every adult you know has done it and been burned. Voila -- WDW is the same.... I've done it and been burned, and so have most parents I know, except the smart ones. hahaha.)
As for a water park -- it would be fun, but again, by day 3, you're going to be pretty tired, and so will the kids. So I'd plan on only a few hours at the water park, and maybe just for the older two or three kids. Sea World, in my opinion, is just too much to try to fit in if you only have 3 days.
Wow, this is a long post! I'm certainly not an expert, but these are some things that have worked for me and family and friends. I hope you have a great trip!