"^^agree, plead guilty and let the solicitor argue it was self defense."
No no no no noooooooooooooo!
There is a statutory defence to allegations of assault and that is one of self-defence. To advance that defence a not guilty plea must be entered and the matter subject to trial. If the defence is successful a Not Guilty verdict will be returned, no conviction is recorded and the defendant is acquitted.
If a defendant pleads guilty then no defence to the allegation can be offered and a conviction results. In short, the defendant is accepting the offence was committed.
As has been mentioned, to give a reasoned answer is impossible without full details, which are not available to us (and probably not to you). The solicitor representing your grand-daughter has those full details including all the evidence against her. Part of his discussions with the prosecutor would involve examining the strength of the evidence against her and the likelihood of a successful “self-defence” argument. He would not encourage her to plead guilty if he believed there was a reasonable chance of her acquittal. If she wants to ignore his advice that is her privilege.