Sorry to hear this. I've just gone through this same process between July 2017 and a month ago when we officially went to court to get the dissolution. I have 12 and 8 year old boys.
They did understand when we told them last summer; hard, HARD conversation. We are both very amicable towards each other, like your situation. She had a lawyer do all the paperwork; I had one review it all, but didn't have one on retainer.
She makes a lot less than me so I pay a good amount monthly for alimony, child support, and split expenses.
Best wishes; PM me if you ever have any questions or concerns. It's a long process and mentally very tough no matter how amicable you are towards each other.