Abby:
Our family used the "crutch" of a stroller for years until one trip when we we didn't bring our youngest and could not believe the ground we could cover when we didn't have to worry about parking it or finding it after riding an attraction! But with that said, when my kids were the age of your youngest, leaving a stroller at home wasn't an option. So which to choose? I definitely don't think a double and a single are necessary, especially if you take the long-standing advice of this panel that recommends you leave the parks in the afternoon with small children for a nap so they can come back refreshed in the evening for the parade and fireworks.
A double would be my pick over a single just in case your 7-year-old needed a respite and you can fit two at a time otherwise. Best of all, you've got a place to hang more packages, store snacks and other child necessities! Lastly, consider the cost of
stroller rental. It may be more economical to purchase one and bring it with you versus renting from Disney especially if you feel you'll need it most when the park closes. Rentals cannot be taken out of the park and that can make for a long walk back to the resort with a child in your arms...
Wishing y'all a magical vacation!
Whitney