Are these small children? I would be sure to have the horse on a line at all times, unless you are in a round pen and the kids seem to be able to stop/turn the mare. Also, I might ask them to sign a release/waiver of liability.
I have done this a fair amount and you'd be amazed at how little it takes to make them walk away sayiing they had the time of their lives! Just like you said; grooming, careful hand feeding , show them what a hoof looks like, teach them how to move around a horse, how to lead. Tacking up (do it yourself, but explain as you go).
When they are aboard, I just have the horse on a line and we work in circles. Just walking mostly, and some trot. You can have them do circles with their arms, touch the hrose's bum , close their eyes, sing a song, count footfalls, stand up in the stirrups, take their feet out.
Do things that are fun, Remind them to look up and relax and to say thankyou to the hrose when she does what they ask.
You'll be great!