If you're budget conscience you should do your own trip
"a-la-carte" & not book a package. It will save you money.
Also, transportation within Disney world is free for all resort guests to anywhere within "the world". If I were staying offsite I would definitely get a car for convenience. I don't like to depend on offsite hotel shuttles. Getting a car would add to your expense. There are many opinions on this but if you want the entire Disney experience you should stay onsite. Plus, having 2 children would make it more convenient for you if you're onsite. JMO!
Here's what I would do:
--Figure out what your budget is & then look into what hotels you can afford. It may be a value resort or moderate. Book your ressie at the resort you can afford now.
--Keep checking mousesavers for discount room codes that come out. If there are any for September they won't be out for awhile, but keep checking often. You could get a better rate on your existing ressie or you could upgrade to a nicer hotel at the same rate you have been paying anyway!
--Also check mousesavers for ticket promotions that occasionally are offered - mainly on Ticketmania. You don't have to purchase tickets in advance so there is no rush to get them. You can get them the day before you leave at a
Disney Store or even when you get to WDW, but you also could get a deal from Ticketmania or another broker before you leave.
--There are many things you can do to save money if you are looking to do a budget trip. You can bring a collapsible or styrofoam cooler in the room for milk, juices, lunchmeat, breakfast items, etc. You can eat your large meal at lunch instead of dinner & many other things.
Ask a lot of questions on this budget board. Right now, my biggest tip would be to do the trip yourself & not get a package. Most of the people on this budget board would probably agree (I think

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Have a great time planning & don't hesitate to ask a zillion questions on these Disney forums.