Assuming they are all complete novices here are some ideas for activities for you to structure your sessions around.
- introduction to 'the horse' including names of parts of horse, and of tack. Reason why tack has developed as it has, and the correct way to use it. He to read a horses body language, basic rules around horses.
- how to mount properly. Incorporate a game where everyone mounts, walks on, dismounts, swaps horses, remounts, etc etc
- walk, halt, steering.
- circles, squares, figures of eight
- Pair them up send challenge them to ride in synchronicity with their pair
- all the gymkhana games you can think of can be ridden slowly by novices
- poles on ground in a square, walk in, halt, dismount etc. Set up fake trail ride obstacles like gates and low-hanging branches.
They obviously think that you CAN do this, enjoy seizing the challenge, and plan plan plan to make it a success.
As far as the welfare of the horses, it is your responsibility to make sure that you don't take charge of them without clear and full instructions from their owners as to diet, management, and what to do in the even of illness or injury. So long as you have this, you should be reasonably clear on how to manage your temporary herd.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.