Obviously we have only testimonies of that from officers and volunteers and confirmation from Israeli authorities,
No, we have the testimony of one IDF member who promoted Palestinian genocide in the past. Everybody else who was on the scene and gave a statement could not confirm that one testimony lie. All the newspapers back-paddled on that story; maybe you should too.
What is there to benefit from faking such document? Is it not obvious that Hamas targeted civilians and children?
Yes. Children are civilians. The repeated claim is that children were not targeted as part of the populace but specifically. In the eyes of most people that would be even more sinister. And Israel needs Hamas and the Palestinians who supported them to look as bad as possible to distract from their retaliation.