Well, you would have to guarantee to never breathe a word of the conversation with the head to a soul, and to have never listened to anyone else in the playground speaking about their own conversations with the Head, or commenting on those of others they have heard about.
Clearly you have engaged in at least listening to gossip, because otherwise you couldn't report it here.
I hope for the sake of all the children in the school your DC is going to, that these are only rumours and that actually the Head has NEVER been prepared to listen to anyone's personal preferences for or against certain teachers, because if the Head has, you will not be the only one asking and a very difficult situation will exist in that school, that is not good for anyone.
The school are professionals and as your child starts the world of school, you have to let go and let them be professionals. This includes knowing which teachers are suitable for Reception aged children. As a parent,you might have views on all kinds of things to do with school, such as who the teacher should be, what kind of discipline should be used, how often they should read, whether DC should sit with X or Y child or not, whether they should have to stay in at playtime because of some particular incident, if they should have to sit in a noisy lunch hall they don't like, if they should have to get changed for PE when they don't like putting their clothes back on, etc etc.........you cannot influence all of these decisions and you should not attempt to. Unless you are prepared to make a specific complaint against this teacher, you really will have to accept the schools judgement on this one.
Whether you go to the ahead and tell anyone the outcome or no-one, any sensible Head will give you the same response. They will smile nicely at you and say they have listened to the information about your DC, but cannot take into account any personal preferences for teachers, that all the teachers are qualified and capable to teach all of the children and you will need to trust their professional judgement.
After you have made this request, the Head probably won't go round bad-mouthing you to the staff, because she will have seen it all before from anxious parents and has bigger, more important things to think about, such as running the school for all of the children.
And you don't know what is best for him at school. Part of children starting school is learning to let go and give over some areas of responsibility to someone else. This is clearly difficult for you and you want to have control over this aspect of his life......but you can't. And any sensible school will make that very clear to parents like you.