I would recommend making the classes fun, and pretty high on the change-up scale. Basically, do a series of activities, and don't let any one activity last more than about 6-7 minutes. This way, the kids won't lose interest in what they are doing. Tell 'em a few stories along the way (either actual martial arts happenings that you have experienced, or myths....maybe involve some of the "heroes" of the MA?), and gear activities toward their age levels.
The important thing is not only that they learn, but that they have fun doing what they are doing. If they don't, then there is no way you are going to get them to learn.....the parents may drag them to class, but you will be fighting them the whole way through it. If they are enjoying it, and it is fun, then before too long, you can point out to them just what they know, and they will be truly amazed.
Do not sacrifice the discipline and respect aspects when having fun, but instead, enhance them. Play a game of seeing who can stand in attention stance the longest....try and get them to laugh, or smile....then they have to sit down.....ever play "Simon Says"?? How about a game of "Sensei Says"? If they are "out" have them do a technique, or a couple of kicks.....
Finally, if these are "tasters", don't fall for a bait and switch. Do the kind of things that students will actually see in class--don't make up something to get them interested and then change the material on them. That won't bring 'em back.
Mainly, as long as you have their interest, you will have them as students. Most kids are not the ones with commitment; it's the parents. The kids want to learn neat stuff while having fun, and the parents want to see their child get discipline (and various other results) out of the arts. Please them both, and you have succeeded.
Peace--