A fill in the blank for the kids would be great. I got one from 2nd graders titled "I love Ms. Jones because . . . " and the kids wrote and illustrated one page each. Kinder kids might need a one word answer. "The best part about being married is ....." But of course, an adult can help them write a longer answer. I still have my book made my my students. But even in just a few short years, I forget their full names, so make sure kids' names are written on their picture.
Another idea - have the kids act out a little 'play' for their teacher. Perhaps a view of what her ceremony will be like. If you do this, make sure to video tape it so that the teacher can take it home to show her husband-to-be.
Even a drawing from each kid of what their teacher will look like at her wedding would be cute. Or what will the cake look like. Or how the car will be decorated.
Or maybe if you do the asile runner, each kiddo could put their handprint on it with paint, along with their name. If you don't do it for the actual aisle, it could be hung somewhere for the teacher.
Good luck. Have fun with this.