The first sentence makes no sense as the phrase "being explained" is used incorrectly. The first sentence is grammatically wrong. You will have to use a gerund in that place.

The second sentence is correct, except you need to use "an" instead of "a" in the phrase "is not a easy task".

The correct way to put it would be "Explaining lessons to children is not an easy task for teachers"

"Being explained lessons to children is not an easy task for teachers."

This first sentence doesn't make correct grammatical sense as usually after a phrase such as "being explained" the word "by" would follow e.g. "It was being explained by the teacher." But if you changed it so the lessons were being explained by the children it changes the meaning from the second sentence.

I would reword the first sentence to "Having to explain lessons to children is not an easy task for teachers."

This gives almost the exact same meaning as the second sentence as the lessons are being explained by the teachers to the children and it's not easy. The only difference could be argued over "having to" means they need to give these explanations but in the second sentence there is no need enforced.

"Explaining lessons to children is not an easy task for teachers."

I hope this helps.

If I want to make this second sentence passive how should I do it?