It also means that with the extra day, during your stay you won't feel rushed to do anything. You can take your time, go back to the room for naps, just play in the room, sleep in late or go to bed early.

If your hesitation is because of trying to fill up the time with an energetic toddler, there are so many things to do, to tucker them (and you) out, that aren't even in the parks. There are splash areas at Downtown Disney for example. Spend a late afternoon over there, for a break.
So many things come to mind.
Relax and have fun ! I think we'd all agree, that if you can stay longer, do it.