Feedback #1
"Long inserters or underground belts should be researchable."
If underground belts and splitters seem too confusing for newbies, then the long inserter is a prime candidate for unlocking. The reason I bring this up is because there's no viable way to input and output resources from one side, which is an important aspect in the game. Belt mixing two ingredients on one side so there's room to output from the other isn't always ideal and is very sloppy when you have to wrap around several structures to accomplish it.
Feedback #2
"Structures can be placed on the left-side cliffs"
Self explanatory, but placing turrets on the left-side cliffs will occasionally bug out the biters or cause irregular spawning of biters on the cliff where they can't get down.
Cliffside
Feedback #3
"Biter waves are relentless and remarkably hard to defend against"
It's a good thing I'm used to biter intense modded games, because this is more like a wave defense than I imagined it would be. Four assemblers needed to automate ammo production to keep up with the thousands of biters coming from both sides. I have a hard time imagining a newbie surviving this.
Suggestion: Have your little companion greatly emphasize having a strong defensive line by having him check to see if there's enough turrets and walls to even stand a chance here. Triggering the final wave objective early can easily overwhelm a newbie, so the additional perimeter check would give them a better chance at survival.
If necessary, have the little buddy construct a tiny defensive outpost on the right side to give them a little head start. He should also point out that radars greatly attract biters so they're not placed in exposed areas as well.
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Other than that, you did a good job there Ben. I look forward to the completed experience, as well as getting my 9 year old brother to try it out. It's unlikely he will survive the last objective, so I'll explain proper defenses to him and see if that makes a big impact on his enjoyment.