Welcome to DC Mitch,
I'm pretty new also, and I've found this site to be very informative and friendly, hopefully, you'll find the same. I'm going to attempt to answer what I can for you, and I'm sure someone will come along and have the rest of the answers for you.
1. concierge-n/a for myself..never did the concierge thing. But, PS stand for priority seating, it's disney-speak for reservations. You don't need concierge to get PS for the the different restaurants, but I think that they can make the arrangements for you if you want. It's not necessary though. PS, however, is usually necessary for quite a few table service restaurants, so be sure to plan where you want to eat and make arrangements ahead of time.
2. you have to arrange your own transportation from the airport to your resort, unless you booked a package that includes it. you can do the MEARS shuttle, which is basically a bus, but, they charge per person. for a family of 4, you might be better off with a cab or private car, because they charge by the carload, not the # of people.
3. In past years, I have seen people swimming in the lagoon, but personally (and especially if I had kids!) I would NOT step foot in that water!!
I'm sure it must not be that bad, but just looking at it makes me kinda leary. I'd stick with the pools, they are usually clean, most have a fun theme, and are heated.
4. This is the most important, and I think most DCers would agree with me- PLAN PLAN PLAN, and when you've planned it all out, PLAN it some more!!:p
I know that it seems like you could wing it with children and just stick to the kiddie stuff, but you really need to get the most out of your vacation, and planning is KEY. It can be overwhelming if you don't have a plan, there is just SO much to see and do and you might not enjoy it as much if you are just winging it. And, you'd be surprised, a lot of kids actually end up liking some attractions, shows, or rides that you might not expect them to. Almost everything in WDW is kid friendly, except maybe some things at epcot, and of course, PI.
5. again, i don't know about the concierge thing, but you have 2 choices for breakfast with princesses, Norway in EPCOT and Cinderella's castle in MK. You need to make a PS for either one. my last trip, we went in September, and tried to make a PS for Cinderella's table 3 months in advance, and still were not able to make it, it was booked that far in advance! I don't know if the EPCOT one is as popular, but either way, book fast!
6. If you book a disney package at a wdw resort, you should get the ultimate hopper ticket, which is fantastic, but it's only good for the length of your stay. regular hoppers allow you to save any unused days. I've never booked tickets a la carte. I found the ultimate hopper to be great. and AK and MK may have MORE for kids, but MGM has some great live shows, an AMAZING fireworks show (fantasmic), as well as fun attractions that young kids might like. You should definitely check it out. The hoppers allow you to come and go as you please, which is really nice, especially if you plan on eating in any of the parks, even if you're not spending the whole day there.
Ok, I hope that answers some questions for you. And just a disclaimer, this is all in my opinion. It's not necessarily fact, and someone may come along and disagree entirely, or point out something that I've made a mistake with. Just keep your mind open, think about your family, and the needs of everyone in your party. It's your first time there, it'll be soo magical! Make sure you take the time to enjoy it and enjoy watching your daughters faces when they see mickey and minnie and everyone else! You'll never forget it:)