Well, I tested it out on my niece and it seemed to work well enough for her, she managed with the advanced rules just fine. In fact the advanced rules quickly became necessary, as the wanted her princess to have powers, which requires the intermediate level rules, and she said she wanted to try and open one of the barrels in the first room, which requires some of the "advanced" rules. I was planning on just doing the first room of the "Basement O Rats" adventure that came with it, but she stayed interested long enough to want to do the next 2 rooms as well, and then said she wanted to make her own game, so I let her do that.
I changed the story of "Basement O Rats" a little, which is easy to do since the story is very simple; just one short paragraph at the start of the adventure, and one equally short paragraph at the start and finish of each room. This makes it easy to customize, adapt, and make stuff up on the fly. I think more adult pen and paper games could learn from this.
I made the story more princess focused, and she seemed to appreciate this. I had made Flame Princess (Adventure Time reference) and asked her if she wanted to play as her, or if she would rather make her own character, and she wanted to make her own (whom she called Pumpkin Princess), so I took her through the character creation process without too much trouble, by asking her a series of questions about what sort of princess powers she wanted to have. She seemed to want me to make a character to play as too, not wanting Pumpkin Princess to adventure alone, so I used Flame Princess as my avatar and we set forth to rescue our princess friend, Ice Cream Princess. Other than that we kept the story line the same.
Since I was only expecting to play the first map, that was the only one I had copied out in pencil. She helped me draw the other maps as we played them, adding innovative features like barrels of ice cream horded underground by the giant rats, and holes in the ground that we needed to jump over. I think my having a character to play as actually helped her understand how to play the game, even though making your own character is normally considered a faux par for the "GM" in pen and paper games (and for good reason) it was what she wanted, and it seemed to work.
I followed the books advice about having the baddies run away when defeated, rather than being killed, but she often declared that she had killed them anyway ¯_(ツ)_/¯
Having finished "Basement O Rats" in two sessions I am quite happy with how it went.
The base rules are indeed very simple, with lots of optional extra rules for older children that can be added on in a modular fashion as and when they are ready for it.
All you need is a copy of this book, at least one d6, a pencil, an eraser, and some paper.
5 extra d6 will make things go more smoothly.
A printer plus the pdf version of this book will save some time if you or the players don't like to scribble things out by hand too much.
Has helpful advice on keeping young players with short attention spans engaged.
No addition or subtraction, everything is done just by comparing dice, a very elegant system.
Uses a grid system to measure distances, so you will either need to draw a grid on a sheet of paper (doesn't take long) or print and play.