You should obviously plead not guilty and put the prosecution to proof. The witnesses will have to attend court and give their evidence in person verbally. They will be liable to cross-examination when their evidence can be tested. The evidence of witnesses not attending cannot be presented to the court unless you agree. The photographs may prove the boy was grabbed but they do not, of course, prove who inflicted them. Follow your solicitor’s advice carefully but you may find that the witnesses to this alleged assault may be somewhat reticent to come to court to give their evidence come the day. From what you say, even if they do appear, their evidence may be dismantled under cross-examination.
You should also keep your solictor informed of any intimidation you suffer in the meantime.