Welcome to our little corner of the world!
His parents are in denial and attempting to shift the blame on you, rather than where it belongs, right smack on their sons shoulders. This is not unusual parent behavior as they are not in recovery themselves, recovery from codependency.
Don't let their words stop you from doing the right thing for you and your children. They do understand the reason, they just are trying to ignore it.
Keep your resolve, you are doing the responsible thing!