I'll just speak to what I did and from my own opinion. We transferred ds from a preschool he'd been at for 3 years (was a daycare/preschool before he was actually in "preschool"). It wasn't a horrible place, but it didn't meet his needs, imo. We were able to use the summer as a transition, and that worked out nicely, but obviously you're not able to do that. You could take dc for a couple of visits to the new school, so he can get comfortable with it, and you could talk about it excitedly. I think the best way to deliver hard news is to do it sensitively and matter-of-factly - "In November, you will get to go to New School for school." It's good that you are going to stay in touch with kids from your old school; that may make the transition easier.
As far as saying something to the teachers, I probalby wouldn't say anything on the way out the door. Especially since it's a small town. Probably, I'd have addressed my main concerns at the time they occurred. If you've done that, then they know why you're leaving. If not, then you haven't burned any bridges. When I wrote the withdrawal letter to ds's school, I thought about listing the things that made me unhappy, but I decided to stay on their good side, just in case things didn't work out with the other place.
As for other parents, I'd just say that the new school meets your family's needs better. You don't have to be specific, if you don't want to be.
It's frustrating that the schools are both bilingual schools! I could completely identify with that frustration. We're not in an area with any language immersion programs/bilingual schools, so we don't have that option. I will say that with just my husband (and me) in a VERY English-speaking community, ds has done very well and is very bilingual. Even if his schools aren't. I think his overall education and well-being is most important in this case.
Best of luck to you both!