I have not read your sitch yet but I will start. In the mean time, my gut reply is act normal and you do not necessarily have to say anything. My kids are 3 and 5 and my W and I have been S living apart since last June. The kids never really said anything. They really do not have a sense of what is normal and abnormal yet.
Me: 42 W: 32 Married 7 years together 8.5 S1: 7 S2:7 Bomb #1: 09-16-13 Recon #1: 11/13 A discovered 04-03-2014 W filed D 05-19-14 but never served me I filed D 12-02-2014 S 05-31-14 Divorced 5-19-16