When the MK gates open, more and more people enter MK. If you wait 20min later, there will be more people in MK, not less. Many people (vaguely) plan on being at MK a RD, but something holds them up. Last time we tried, here's what happened: We went to the bus stop and saw a long wait. So we ran to the WL boat dock and waited for a boat. Five minutes later, the boat arrived, but the captain announced there a stoppage due to fog! So we went back to the bus stop, and waited another 20 for that bus to finally arrive. (no, we didn't miss a bus by going to the dock.) At MK, the early am bag check/park entry is now sometimes rather slow.
I'm not trying to flame you, but I honestly don't see the point. IMO, MK looks very creepy when it is empty. I went to MK in early 2002. Seeing MK that empty is something I've tried to erase from my memory.
In addition to crowds, the weather is always another big unknown. An extended family member once attempted a beachfront picture like this - not at WDW. She paid a handsome sum to a pro photographer, but the weather that day was terrible. It was cold and very windy, and none of us look good or happy in the pictures.
Last, you simply can't predict how young children will act in any given moment. Many items over, I've seen - more or less- the opposite of what Yulillin3 describes. time and again, I've seen parents with young children urgently trying to get a perfect picture in front of the castle that never happens.
I'm not sure how to say this, but going to WDW with high expectations of perfect moments often turns out to be a recipe for disaster. If something goes wrong that day: slow transportation, bad weather, baby drool/tummy gas/burped up milk.....
I'm not saying don't try to arrive for this picture, I'm saying: go in with an open mind that it might not work the way you think, and keep your options as open as possible as long as possible. Keep both days as options, but don't let it ruin your vacation if things don't work as planned.