Ask your solicitor for the minimum/maximum based on similar cases he has undertaken. Ask him again when you know who the magistrate is.
There's obviously legal set penalties but some Judges are more lenient than others. Just be honest in Court. Admit you did mean to kick but someone moving meant a kick (maybe to the upper arm) in frustration resulted in a kick to a cheek. Admit you were guilty and admit liability. If you can see a Doctor NOW for anger management so at least in Court it's noted that you are getting 'help' and your fine may end up a whole lot better.
You worry me. You have shared the problem here for advice but you don't come over as being either penitent or remorseful - just worried about how it will affect you and what it will cost you.