S.,
I am a speech pathologist and also face this concern with my five year old and newest baby. My second did not suck his thumb or take a pacifier.
I can tell you, having tried with my oldest and my newest baby (just four months) to make them take a pacifier instead of their is impossible. You wouldn't have prevented it even in hindsight. I most definitely would have preferred them to have the pacifier, and I assure you I tried because, as you said I could have weaned that.
Pam Marshalla, writes a very parent friendly book on ucking. I would obtain a copy of this book and employ some of the strategies she talks about.
What has worked for us: Setting a timeline of big days and curbing the occurrence of the ucking. When turned 3 she could only suck her thumb at home. When she was 4 she could only do it upstairs. When she turned 4 1/2 it had to be in her room. And now that she just turned 5 she can only suck her thumb on her bed.
This doens't make her thumb as something that we won't allow it just puts limits on her when/where she can do it. Sometimes, when she just needs a few moments to herself, I will find her in her room and on her bed.
As for affecting their palate/teeth; you really don't have to worry until their grown-up teeth begin to come in. Until then, you have some time to wean them.
Because your children are a little bid older you may not have the luxury of time like we did. But you can use the holidays as milestones...after Valentine's Day you can only suck your thumb at home. By Easter only in the living room and your room...etc.
Good luck!