This depends on two things.
One, how old are your grandsons?
Two, did they cheer up at your reasonable explanation?
If they are small children, or were unable to feel happier after your explanation, then I would have said something along the lines of, "Son, I may be shoving my oar in where it isn't wanted, but the wee lads were badly upset. They actually told me you and their mother had been shouting and yelling and that you had pushed her into a pond. I think you need to reassure them."
I would not call pushing someone into a pond phsysical abuse, unless the pond was deep enough for her to have drowned in - it could have been sheer exasperation or erotic foreplay, but should not have been witnessed by (young?) children.
So yes, speak to your son, at the risk of offending him for your grandsons' sake if not for your DIL's or his.