I wouldn't schedule a whole day or two for a down day. Keep in mind that even on your 'down' day you are going to need to find a way to keep that 2 and 4 yr old entertained and even the best parents can only take so long in the tiny value rooms with thier kids and not much to play with before you go bannanas! Keep in mind your kids will KNOW by then all the fun things they could be doing instead, don't know if the 2 yr old can talk but out DS at 4 drove us nuts wanting to go do this and that instead of just chilling.
I would pick 2 half days as down days, one sleep in, swim, chill in the room, etc and the other come back during your mid day break and don't go back out that evening. Rest and relax.
I would rather have a 2-3 hour break everyday than a whole down day, I think that is more beneficial, espically the time you are going, Sept is still hotter than the surface of the sun most days so the daily break is awesome. With the price of tickets for that 9th or 10th day you could just go eat lunch and ride both the 2 yr old and 4 yr olds favorite ride and still get your money's worth for that day.