It's
been proven again and again that children have the at the very least a
partial ability to separate fact from fiction as early in their mental
development as age 3 -- that's barely a toddler who begins to make and
understand meaningful sentences.
On the other hand, prominent scholars
like Richard Dawkins put forth that the belief in non-real entities like God,
Santa Claus or unicorns is an evolutionary adaptation by which small
children tend to simply accept as real whatever their parents (or
figures of authority I would say) tell them is real, as a mechanism to
facilitate learning in general. Unlearning or reclassifying as non-real something that you've been taught was real by your parents or authority figures is a cognitively demanding operation that may not seem advantageous in the immediate context of one's life, so it's not something that must eventually inevitably happen.
So the whole point is that, indeed, parents today, like they have always had to do, should get involved and make sure to tell children the truth, and to point out when something that's not real is presented to them as real. But there's no need to make a big drama out of it -- the kids get it.