My kid was in before care too, but the first day of Kindergarten, I followed the same routine as other parents whose kids weren't in before care. I took him to school a little early, allowed him to play on the playground, and then the teachers came outside and called for the students to line up, and I walked him over to his new teacher's line, then I stood with the other parents while the kids left us and walked into the building together. Then the parents stood around for a few minutes and introduced ourselves to each other and then we dissipated.
The next day and every other day, the before care staff walked him over, and then made sure he got into the right line, but the first day I wanted to be there, and was luckily able to make it work. If I hadn't been able to he would have been fine with before care, but I treasured the opportunity to do it. I'm a teacher too, but luckily we had a different first day, so I could do it.