Is there really anyone left in the country who thinks divorce is a scandal? Even the Queens' children have 3 out of 4 divorces and I'm sure your in-laws have lots of friends whose children have divorced.
There's no reason for you to explain anything, and they probably have a good idea anyway. Once you start justifying yourself it sounds defensive.
Co-parent has given excellent tips on sticking to the facts and keeping your language neutral when briefing your MIL on watching out for the children.