What does common sense tell you? Or is that asking too much?
If you had been walking by and seen this happening, what would your response have been? Would you have gathered a group of this woman's peers so you could discuss what was going on and then make a decision? Or would you have turned and walked the other way? You've already effectively said that you wouldn't have stopped her.
If I had been walking by, I might have seen better and heard better what happened. That means I might have known something and been making a decision whether to intervene with reason. We have none of that, just a poor quality video seen long after the fact.
It is not just a saying but a deeply important principle that one is innocent until proven guilty.
If that kid has no serious injuries, they will have a hard time proving their case and it would be terrible if the kid was traumitized in the meantime by foster care.
This decision is always tough but I must give the benefit of the doubt to keeping a family together.
Now, let us assume another set of facts. What if a medical exam shows serious and continuing physical abuse by someone. Then it is appropriate to hold a court hearing on an expedited basis to review that evidence (with parents notified to be present)and let a judge decide if there are exigent circumstances. Another relative should be the favored temporary custodian if that is the case. But that depends on a lot of facts that don't exist or are unknown to us in this case.