I went through this when mine were 1 and 3, very close. I did clearly explain the new agreement with my 3 year old. It was hard for me to leave the 1 year old as he was very much a momma’s boy at the time, so my ex and my older boy got some alone time in the beginning. But once my little one got over the separation anxiety phase he seemed to take to our new situation just fine. Having his older brother around helped a lot I’m sure.