Why would you explain the solar polarity flip to a 5 year old

Why would you need to?

You wouldn't need to, and they would never get it. That is more for 7th & 8th graders.

What? That does not sound like a good idea, even if the child is a super genius junior rocket scientist. For children, even just explaining the North and South pole is pretty complicated stuff, except as Santa's home. And since it has not been any Earth shattering thing, well, it's not really that important to try to talk about it...right?
But anywho, for a simple explanation I found this one:

February 15, 2001 -- You can't tell by looking, but scientists say the Sun has just undergone an important change. Our star's magnetic field has flipped.

The Sun's magnetic north pole, which was in the northern hemisphere just a few months ago, now points south. It's a topsy-turvy situation, but not an unexpected one.

"This always happens around the time of solar maximum," says David Hathaway, a solar physicist at the Marshall Space Flight Center. "The magnetic poles exchange places at the peak of the sunspot cycle. In fact, it's a good indication that Solar Max is really here."

Why on earth would you want to. let him be a child and enjoy life instead of filling his head with useless garbage. he should be out swimming running / playing with other kids and being normal.

You wouldn't.

Since they can;t comprehend those things, the idea is more likely to be scary than informative.

I would not, as 5 years is too young for that kind of abstract thought.

You wouldn't