Yours is a fascinating question because it raises a further question: What is the nature of wrong-doing. Let us first discuss that and then turn to your question. I want to examine the issue of wrong-doing first because forgiveness takes place in the context of another’s (or others’) wrong-doing. If we find no wrong-doing on the part of your daughter, then forgiveness would not be recommended.
For something to be morally wrong, we need to examine three issues: the act itself, one’s intentions in performing the act, and the circumstances surrounding the act.
The act itself of yelling out in a public place is not wrong when, for example, a person is being robbed. That circumstance of robbery makes the act of yelling good because it may prevent the wrong-doing of robbery. Thus, yelling in a public forum is not, by itself, an unjust act.
Your daughter’s intentions are not likely to be morally wrong. Given her autism, we can induce that she is not trying to cause trouble by embarrassing you or by harassing customers in the store. Her intentions are likely a response to something uncomfortable within her or it could be some kind of a conditioned response to something in the environment of which you are unaware. Thus, it seems reasonable to conclude that your daughter’s intentions are not morally wrong.
The circumstances, of being in a crowded store, do suggest decorum, but again we have to factor in the circumstance of your daughter’s autism. Her autism is likely to contribute to her behavior much more strongly than norms of conduct in public places.
When we examine the act itself, intentions, and circumstance, it is clear that your daughter has not engaged in wrong-doing. Thus, I would not recommend that you forgive. Instead, I recommend that you understand her action in the context discussed above. You might consider practicing acceptance (for now) of her actions that embarrass you (as you have stated). Further, you might take steps, through behavior modification techniques, to condition her behavior toward not yelling in public places by rewarding her for quieter behavior when in such places.