I've been running D&D games for kids in a higher grade than you (mine are aged 13-18) at a friend's public library, but some of it will carry over I feel.
- Whatever you run for them, make it be simple and a one-shot
Seems kind of obvious, but I cannot stress this enough. My kids have trouble focusing on the game when they just have to do simple things (getting gear, marching orders, combat, etc.).
- Get sets of die for them to roll
My friend found a set of simple colored die that are translucent for around 25 USD and she keeps them stored in a little tackle box behind the desk. When the kids show up for the game we pass them out. When they finished the initial program (it was a trial run during the summer) they got to take them home with them.
- let them look at the various classes, but restrict their races and stats to the common ones
We used the standard stat array from 5th edition (15, 14, 13, 12, 10, 8) for character creation, and kept them confined to playing as Human, Elf, Dwarf, or Halfling. The other races are nice, but throwing the book at them right at the start might overwhelm them.
- If they ask any questions, answer it and try to point out where it is in the book they can refer to
My kids ALWAYS forget about their class or racial features, what they have to add together for attack or damage die, and even what spells they have. Be patient with them, and if there's a rules question they have answer them as best you can, if possible, with a section of the book to show them a larger breakdown of the ruling.
- Sessions will be short, prepare for it
I have 2 hours for my group and I struggle to get them through the session I have planned out in time, simply due to the out of game shenanigans. Have it be a very simple thing, maybe limiting it to 1-2 dungeon rooms a club meeting.
- Have fun
When you have fun with it, it will carry over to the kids and get them involved and have fun with it.