Just a small point of clarification here.
Monster Slayers isn’t aimed at 6 year olds. It’s aimed at gamer parents of elementary school aged kids, for intended use by their kids.
If this was a product advertised to children where it would be seen by them, then that changes that assessment. Right now, I don’t think so. It's not a small point. Food for thought.
I love the cover art of the Chimera on the Monster Slayers book by the way.
As for Art Direction in D&D, generally, I wasn’t much a fan of the steam punk look to 3.xx and to be honest, most of Wayne Reynolds oversized anime sword stuff in Pathfinder is very off-putting to me too. Albeit, it at least Pathfinder Core Rules' interior art has the virtue of being a consistent art direciton, which has its own rewards and aesthetics, I suppose. I do approve of that approach, just not the stylized anime stuff.
Wayne Reynolds is a good artist – but I REALLY don’t like Anime styled exaggerated fantasy art. I have the 4E core but have never really looked at it to judge its artwork. Generally, I did prefer the layout approach in the 3.xx core.
Meh. All by way of saying: GET OFF MY LAWN!