The outward appearance of civility between your daughter and son-in-law is not the peace which passes all understanding. That false peace, that strained silence of an armed truce, is often merely outward. Men put on the air of peace when they do not feel it in their hearts, and this is as stupefying as opium, for the devil himself can lull a household asleep while the storm rages within. My design would be to disturb that kind of peace if it is a peace bought at the expense of truth and holiness, for sin brings forth strife. The God of peace never tries to establish a peace between good and evil, for that would be monstrous even if it were possible.
The war in that house, the sharp words, the cold distance, springs from a deeper conflict. There is going on in every human heart a strange conflict between life and death, between the Spirit of God and the spirit of evil. Since these two must fight, their children within us continue in conflict. This explains the strife, but it also points to the only remedy. To do the will of Jesus, this is rest. To know the will of Jesus, and to do it, is to abide in the peace of God. This is not a peace of apathy or levity, but a divine peace born of the Spirit, a fruit of sanctification where holiness becomes the mother of peace.
Yet where is such peace to be found? We do not cry out for more willpower, but with the Apostles we must look to the Master and pray, increase our faith. The Lord Jesus Christ is our peace, not as a mere gift He dispenses, but in His very Person. He is our peace. He has made peace by the blood of His cross, ending the strife between man and his offended Maker. If that household is to know a moment's true calm, it will not be by an outward treaty negotiated on their own terms, but by each soul coming as a guilty rebel to the Prince of Peace. The blood of Jesus whispers peace within; take your full of peace there, for by that sacrifice a covenant of peace is sealed.
Your worry for the little ones is natural, but remember that the God of peace preserves peace when it is made, and is the restorer of it. Bring these grandchildren before the throne of grace, and plead the promise that the Lord of peace Himself give you peace always. He can always give peace, for He never changes, and He can command peace in that home, in those young hearts, even when the winds above are raging. May the Lord bless you with an abundance of that peace which keeps the heart and mind, and bathe you in seas of heavenly assurance. The Lord of Peace Himself give you peace.