Kirtan Sohila

In an effort to focus on the question:
Importance of reading SOHILA at bedtime and how to explain it to the children?
Of course telling them that it is important we need to do it, is not working with the children, Neither is the argument that Bhai Gurdas Ji or someone else says so.

There is no quote from Sri Guru Granth Sahib that answers your question directly.
However, consider:
What is the purpose of reading the SOHILA or any other Bani?
As best I understand, it is to understand the message of the passage at hand and implement it in one’s life.
So reading any Bani at any time for the sake of reading it has NO VALUE.
Remeber the Gurbani Quote:
ਰਾਮ ਰਾਮ ਸਭੁ ਕੋ ਕਹੈ ਕਹਿਐ ਰਾਮੁ ਨ ਹੋਇ ॥ਮ:੩, ਪੰ;੪੯੧)
(Everyone repeats RAM RAM, but saying so doesn’t achieve the aim)

So the answer to your question is first have your objectives align with Gurbani message then try to get your children to understand them and adopt them.
The important thingis that they understand Gurbani and live accordingly,
daily ritualistic recitation has no value.

The daily, not just daily, but at all the times possible, reading of even one paragraph of Gurbani, with the intention of understanding the spirit of the message and adopting the message is highly recomended.