Ive been charged with assaulting my son because he told a teacher…

If you have just been told by police of these allegations, and they have not yet actually charged you with assault, then they will likely ask you to give a statement. Usually there is nothing to be gained in giving such a statement. If they think they have enough evidence to charge you, they will do so, regardless of what you might say. But it would be hard to convict on just the testimony of an 8 year old if there is no physical evidence or other witnesses to an assault, and there is some possible explanation ( a messy divorce) as to why he may have said untrue comments about you. So your giving a statement could just give further evidence to try and convict you.

What I meant by your son testifying is that if you are charged and there is a trial, they would have him appear in court to explain to the judge or the jury what he says happened.

If you are charged this is potentially a serious charge and you should contact an experienced criminal defence lawyer. You do not need one perhaps if you have not been charged, but if you. Speak in person with one you will almost certainly be advised to give no statement to the police and not to answer any questions of the police. You have no legal obligation to do so.