Grade school age kids and playing RPG's can be a bit tricky.  I know. My son and I play them on occasion. 

The biggest hurdle is pretty obvious in my son's case.  His attention span.

He cannot sit thru a long, tedious session of combat rounds, especially if it's just boring dice roll hit and be hit. 

Thankfully, there are two games he really likes.  Tiny Supers, and Amazing Tales QuickStart. 

Tiny Supers is our favorite RPG right now.  It's part of the Tiny-D6 set of games.  Roll 1D6 for things that hard, 2D6 for medium difficulty, or 3D6 for the easy tasks.  You don't add them up.  You just want a 5 or 6 to come up.  For statistics, that's a 33.3%, 55%, or 70% chance to succeed.  You get two actions on your turn. 1-handed weapons do one damage, 2-handlers do 2 damage and go on a cooldown until your next turn.  There are classes, but no stats beyond 6-8 hit points set by your class, and all you do in character creation is choose 3 abilities from a list.

Amazing Tales QuickStart is free, and meant for very young kids.  It's super free form.  You just pick four things your character is good at, then assign a spread of a 1D12, 1D10, 1D8, or 1D6 to a skill.  It could be sword fighting, being friendly, teleporting, piloting spaceships, summoning a rainbow, whatever.  It's supposed to be a kid's imagination gone wild, and so long as you can make a decent argument that your ability applies, you can roll for it.  A 1 or 2 is a fail, but 3+ is a success.  A fight can be a single dice roll if you want, but typically I ran it as about two or three dice rolls.  A beginning, middle, and and end for a fight. 

I am curious what everyone else plays if they run games for kids.

Sometimes I can get my son to play, He just turned 8 last week. I have gotten him and a friend into Melee, and then a programmed adventure from Dark City Games (adapted from The Fantasy Trip), which is nice because their choices are dictated, and we play on the hex map with the counters. They enjoy it and play can last up to an hour or so, sometimes less.

I am about to introduce them to the game I really want to play, Enlightened from D9 Games, which moves even faster and I can whip up monsters on the fly if I have to. Also, the d9 concept is fun and will engage them more, I am hoping. My goal is to make that game feel as much like Moldvay D&D as I can, which I also may introduce them to at some point, just because it's how I started. But right now these other games are where my interests lie.

it isn't always pretty sometimes they bicker, and goof around, and my son endeavors to put figures that he fancies on the board etc. But at least they are at the table for a bit.

My son's 25 now, but when he was little I tried a few simpler RPGs with him - Faery's Tale and Monster Island were ones he particularly liked.

I have a old page on RPGs for kids that has a bunch of broken links now.

https://darkshire.net/jhkim/rpg/whatis/kids.html

When I was in elementary school I liked AD&D 2e, and would have liked other complex systems had I had the money to buy them. Now admittedly I was around 10 when I started playing, and that might make a significant difference compared to 8, but my view has always been that kids like a great deal of detail and crunch, and it's adults who have obligations who prefer the rules lite systems.

Because when I was in elementary school there was a phase where I was playing D&D every day. All the detail and fiddly bits are interesting in that case. Now I'm doing a few one shots a year if I'm lucky. So now I want a system that's stripped down enough that it doesn't get in the way, but has just enough to support that single 3-4 hour session.

That said, my mom was a tutor (retired now), and she noticed a massive change in kids born after 2000, that they somehow can't do very basic calculations. Something went wrong with how math is taught in school. So it's possible with this sharp drop in math ability, that the same games I found fun and challenging would be overwhelming for today's kids. But if that were the case I'd be putting extra effort into making sure they were at least somewhat as good at math as I was at that age, and I'd be looking at what I could do to use RPGs to help improve the math ability.

Also, apropos attention span: Reading good, old school poetry, with rhyme and meter, is great. I tutored a few kids, and had one who couldn't even finish a sentence while reading. I got him reading Where the Sidewalk Ends by Shel Silverstein for a couple weeks, and then switched back to Pokémon and Geronimo Stilton, and his attention span improved dramatically. Good poetry makes you want to keep reading, and that's something that has also been abandoned in schools in recent times. The rhythm and rhyme will get you into the habit of reading longer sentences, and once it's a habit you can stick through longer sentences in prose (provided it's a story you're interested in).