Hello, John!
Welcome to the Disney Parks Moms Panel! We are delighted to answer your question concerning your upcoming Walt Disney World vacation!
My general rule is to always go with
Extra Magic Hours, however, since you have two infants, my vote is to visit
Magic Kingdom during the week when the crowds should be lower. I have had a tired and screaming baby in Magic Kingdom at 11:00 pm, and it is not that fun. If you did want to visit that Saturday and take advantage of Extra Magic Hours, you could split your family where one adult takes the infants back to your resort and another adult stays with your older kids to experience Magic Kingdom with very low crowds!
With small kids, it is ALWAYS best to plan on arriving to any Disney Theme Park when it opens. You will have at least 90 minutes to two hours of low crowds before the late sleepers start arriving. You will be amazed at how many attractions you can ride in that amount of time!
If you have other questions, please let us know! We are here to help in any way!
Have a MAGICAL day!
Wes