Hi, Sarah! I just returned from a trip where my youngest two sons and I tagged along with my husband for a conference he was attending at one of the WDW resorts. Therefore, I am fresh with advice for you!
Since you are staying off property, you need access to a car, and if you're not driving, you'll want to rent one at the airport -- to avoid shuttle or taxi charges. For planning your days, set aside travel days for non-ticketed experiences such as Downtown Disney, the Boardwalk or Fort Wilderness. Or, you can make
Advanced Dining Reservations for a character meal such as Chef Mickey's (Contemporary) or 1900 Park Fare (Grand Floridian).
If your husband's conference includes meals, you'll need to take this under consideration. Also, check with the meeting planners about discount tickets to WDW - after-2 p.m. or after-4 p.m. tickets may be helpful for you or your husband.
Remember to pack your day pack with rain ponchos, sunscreen, bottles of water, a camera with batteries/memory cards, cell phone, a few snacks, hand sanitizer, baby wipes and possibly an extra change of clothes for each girl. Bring an umbrella stroller for your toddler (one with a canopy and a cargo basket), but hold off on bringing a double. It's just easier to navigate through the crowds with a single stroller.
Remember to take each day slowly and take frequent breaks. Even though it's a bit more expensive, consider dining at table service restaurants without buffets. In a crowd, it's difficult to manage a tray, a toddler and a little one in a crowded quick-service restaurant or for a buffet. Plus, it gives you a break from the pace.
I hope this helps you plan, and if you have more questions, please let me know. Remember to take lots of pictures so that Daddy isn't too jealous of all the fun you're having without him, and keep him posted throughout the day. Have a wonderful day and a magical trip!