When my oldest was in Kindergarten, a neighbor and I swapped every Tuesday. One Tuesday morning I would volunteer in the classroom and she would watch our 2 year olds. The next week she went in and I took the little kids. However, each of our Kindergarteners were pretty clingy, needy, and whiney when we went in. They had a hard time focusing on their school routine when Mom was there too. When it was the other Mom's turn, our kids were perfectly behaved. So we realized as much as we loved being there, it was probably more for the teacher, the other children, and for us than for our own daughters' benefit. The next year I only volunteered with field trips (if I had childcare for my younger, and I didn't always either so I couldn't volunteer for everything) and went in for programs.
Why can't you take him to the birthday parties? Take the 2 month old with you if your nursing. Why can't the other 2 stay with their Dad while you go once in awhile?