Maybe when you change your voice, it could be to one of excitement and adventure? Help them to think that this is a great thing-something exciting to face! Because you already know you will be facing a challenge with each change, maybe your own apprehensions can be heard in your own voice. Maybe also, you just don't need to mention the changes. And when they notice, don't explain it to them-just be excited if you can, or tell them, well, that's how life goes sometimes! But all in an upbeat manner.
It may not be helping to get down on their level because at their level, they are not adjusting to change, and sometimes are throwing fits due to it.
I read a book by Kevin Leman called Have a New Kid by Friday.
Basically the book is simple, and he talks about the fun things we do with our kids, or when they are expecting to go to a friend's house or something.
When they misbehave, or ignore our requests/rules in the home (say it ONCE), and the time comes when you are supposed to go for your weekly ice cream treat, or visit to a friends, etc, when they approach you and say, Mommy, can we get ice cream today? Or, can I have a cookie? Or, are we going to Tommy's house today?, you say, No, we're not going to Tommy's today/getting ice cream this week/no, you can't have cookies today, and the LEAVING THE ROOM. He says the child will follow you and ask WHY??? And that is when you tell them their behavior was unacceptable, or whatever the problem was, and then leave the room AGAIN. Don't argue with the child, don't continue to explain, don't comfort them when they start to cry about it. Just ignore their reaction.
When they promise to never throw a tantrum again, let them know you appreciate that, and that it's great. And when they say, Can we go get ice cream now? STILL say no this time. Do not change your answer. But Mommy, I promised I won't do it again! Still do not change. This shows the child that the truly won't be rewarded when their behavior is unacceptable.
The next time you face the same situation, their behavior will probably be different because they realize Mommy isn't going to coddle them in every small situation.
Since you are married, it doesn't sound like you have an unstable home life that causes them to fear change, as I've seen some children act that way after their parents separate. Since you have twins, it may work very well to follow Dr. Leman's advice, and still reward the one who behaved, yet show you mean business with the one who misbehaved.
Some may think it sounds heartless, but sometimes even tho it's making Mommy's heart hurt on the inside to discipline them, because we want them to be happy, it's actually a huge help to let their little hearts ache a little (as far as proper discipline) so that they will learn to be content without having to throw a tantrum to get to a place of contentment.
My own children were past this stage by the time I found this book, but it does sound like good advice to me.
On Amazon, out of 149 reviews, 132 of those reviews are 4 or 5 stars (out of 5), 106 of them being 5 stars. Many people say that this book has saved them. Please read the reviews, and hope this helps you! You may even decide to buy the book for yourself. He's better at explaining his book to you than I am. =) Best wishes!
http://www.amazon.com/Have-New-Kid-Friday-Workbook/produc...
This is the link to the workbook because there were more reviews to read. I only had the regular book, which was also good, in case you would like to purchase it.
(ps. I don't think I came off in this response as harsh in anyway-if I did, it certainly is never my intent to do so. We're all in different stages of our lives, and we are all trying to do what we think is best, so I am not judging or ever throwing stones. The good Lord knows this mama has had her share of times of needing help when I can't figure out the solution!)