The question is Help me to convince my son that Santa Claus exists, scientifically. That question was put on hold as too broad.
That question seems straightforward.
The manner in which it was asked could be argued by purists that it is off-topic, but it could very easily be made on-topic by said purist just by altering the way it is asked.
The question reminded me a lot of Santa's broke. How much does it cost for someone else to be Santa?. In fact, that question also asked multiple questions in one. It more cleverly crafted the overall question to make it all sound like one question, but there are multiple in there. The question at stake here is likewise one primary question for which you need to consider multiple points.
The question already has one good answer, though I think we can do better.
There is an answer-in-comments which, if it were allowed to be posted as a full answer, would make for a great answer that would actually solve OP's issue in a way that would reduce or eliminate the lying.
The asker stated that they thought the question might be off topic, but again, it only is because of the manner in which it was worded. The question itself fits well with the style of many other well received questions on WorldBuilding, excepting the specific wording used.
It would be nice if someone who thought it was too broad would at least try to point the asker in the right direction to fixing it up. Please leave a comment with constructive criticism that would allow the question to be fixed up and re-opened.
So is that question really too broad? And, if it is, what do you want to see adjusted for the on-hold to be removed?
For the record, I strongly disagree with what that parent is doing, lying to their kids about Santa. I have told mine the truth from the beginning. But I know that lying to your children is the norm in this situation, at least in the U.S., and that I am in the minority.