Should I say something?

I would. E er describe pushing someone into a pond as erotica foreplay either. Could have seriously injured her had she hit her head on a rock

Too many adults turn a ‘blind eye’ when children reach out for help ….. your grandsons have reached out ….help them!

Another thread where the op never returns?

Were the GC serious, upset when they described what had happened? If yes, then speak to your son telling him the GC are worried. You don't need to go in raising the violence part but you could ask your son if any violence ever occurs. Give him the chance to come clean or explain about the push into the pond. He and his wife need to know how the GC are reacting. If it wasn't quite how your GC have reported to you at least you will still have made the parents aware of their behaviour affecting the GC.

You do not give the child away, because the father may turn vicious with him, too. But you can say that you notice the children are jittery and parry you when you speak about life at home. If he injures her seriously, or even kills her, you will know that you could have done something, but did not. You can even file an anonymous report with the police. DV is not ever to be ignored.

I wouldn’t ignore it. I would point out to the parents that if the children were to tell anyone other than yourself ie a teacher or neighbour, the matter might end up being reported to social services. Reports are often made by neighbours who have concerns.

It's taken GREAT courage to do what these children have done. Speaking out is very hard. To not be taken seriously, or feel disbelieved after any disclosure is - I can vouch for this - a lifelong betrayal of trust.
It's takes equally great courage and heartache to do the right thing, to stand by what they have said, but it's you that has to be the strong adult on their behalf. Protect them.
If it amounts to nothing then you will still have done the right thing in being their loving advocate.