OP I've read the full thread. From what you've said, this isn't self defence on your daughters part.
Working in security, you should know the meaning of self defence. It has to be reasonable force used when there is an immediate threat of/ reasonable belief of harm and the person defending themselves has to attempt to retreat following an initial strike. Force can then be increased if the risk of harm remains/ is heightened. It isn't self defence if the initial perpetrator is moving away/ risk of harm has been eliminated.
If the footage doesn't show the boy with the stick in hand/ dropping it when your daughter reacts (which it would if your daughter was acting in self defence) or she continues to strike without attempting to retreat, then she has committed an assault.
If the boy was, for example, tickling her ear/ hair with the stick and she's then turned round and punched and kicked him, the force she used wasn't reasonable in those circumstances.
If he was hitting her with the stick in an aggressive manner, a punch OR kick would be reasonable in those circumstances. But both wouldn't be necessary to eliminate the threat and either would give her time to retreat.
The footage shared online by the boy or parents, prior to any police investigation, could be argued as hindering the investigation. Your solicitor can advise on this more.
Please do not collate any evidence yourself in the form of witness statements, this can go against your daughter. Provide the police with names of witnesses and let them take statements.
As others have said, a voluntary interview is an interview under caution. The only voluntary part is you get some say in a date and time of attendance. These are normally used when there is evidence of a crime occurring, but not enough evidence to warrant police attending a location to arrest (crime that isn't in the public interest/ no immediate risk of harm). Your daughter can be arrested upon attendance, or if she tries to refuse the interview/ leave the interview without following the correct process.
I work in the criminal justice sector, for reference.
Good luck for Sunday.