Children can handle a lot of fantasy - after all, they create fantasy on purpose, by play. Play is not play, it is mimicking adult behavior. Children know very well they're not doing things for real, after all, they're not big.
However, as OP says, a time spent by fantasy fun is a time missing from the reality fun. A time spent in unreality could count as a brain damage. Child brains are developing to adapt to anything they see and do. If they grow up adapted to live in fantasy worlds, they will have a brain badly adapted to live in a real world, not wired to solve problems, delay gratification, overcome frustration, foresee dangers and most importantly, have critical thinking.
I believe this is the basis for ADHD, children who spent too much time in front of TV, watching literally insane TV programs have damaged brain, frantically high-keyed in a way that real life does not work. And as it probably happens the most on the East coast of US, the ADHD rates grow the closer they are to the East coast. Every 30 to 7 minutes there is an interruption of advertising. TV wires the developing brain in a way that shatters the ability to focus for a longer period. If that's not brain damage, I don't know what is.
Childhood has to be fun. But parents think that fun is by definition useless, so childhood has to be useless, or it's not fun. And if there's any time to be useless, let it be in childhood. That's how they think. This is of course not necessarily true, but it requires some self-education and good learning materials to have fun in an intelligent way. And of course take children to Camp Quest, a secular camp that is a lot of fun and teaches critical thinking, and of course there's no mention of religion. If you spent your life watching Christian monopoly on child, youth and foreign language camps, you can appreciate Camp Quest.
However, I'd make allowance for science fiction books! Although I got to sci-fi fairly late, I think it gives me a great understanding of experimental sociology, the most important topics, impact of new technology on society, dealing with alien or stranger cultures, love of science, immunity to religion, liberal view of women, and so on.
Comics are not sci-fi. Superheroes are idiots in spandex who beat up people suffering from social pathology, but never change the system that produces them. (Batman has at least some idea of the evils of capitalism, but offers no solutions)
I also greatly appreciate Jacque Fresco's insight into parenting. He has the same opinion as the OP. He talks a lot how he taught his children problem-solving and how he could communicate with them when they were just a few months old. Fresco was great with children and it's good to see I don't admire him for nothing.
That reminds me. Is there a way to learn child psychology or get an experience with children? I'm a guy, but white as a chalk and college-educated, spotlessly clean criminal record, but I don't think there's an opportunity in our society to study children directly or indirectly. I don't have any young relatives and TV shows I've seen employ too much artistic license and stories. I have read some sociological studies, but they were just a text.
We all agree that parenting is a great act of philosophy. Also a saint's trial of patience. Let's say I wonder if I can stand up to the challenge. I wonder what kind of a parent I'd make. frankly, I haven't seen a real live child for more than 15 years and I wonder if there's a way to prepare.