S.W.
I moved out of our home when our daughter was 4 1/2. She already knew things were not right between mom and dad, I'm sure your son knows, too. Let him ask questions and answer them as honestly as you can, without making this anyone's "fault". Knowing the truth is much less scary for kids than not knowing.
Your one year-old will absolutely "notice" and will need extra attention and reassurance, also (I moved in, over 20 years previously, with my now ex when his daughter was one and she had much more difficulty adjusting than our 4 year-old has because her parents were always fighting viciously).
My daughter has not needed counseling, but if you feel that would be helpful, then absolutely find a counselor to work through this major change. If parents can be civil and respectful with each other, and truly act in the best interests of the child, the child can adjust to separation and divorce.
If you can, try mediation before lawyers. Lawyers make more money the more you two fight. Mediation can give you a chance to work things out the way that will work best for everyone.