SOS - need suggestion - desperately

Hi...
Both of my sons had math course in the Step Ahead, Discovery Park, Tseun Wan, for 7 and half months. They are having a lesson at the same class, although they are in a different level. Until last week they came home and told me that the substitute tutor, Emily, PUSHED MY OLDER SONS HEAD and shouted SHUT UP to him. The reason was because of my older son told his brother the answer that asked by the tutor. So the tutor shouted SHUT UP to him and pushed his head. I think this is PHYSICAL ABUSEMENT and it is NOT ACCEPTABLE.
We approached them by trying to make appointment with the owner (this is a small centre). But, the owner was never showed up. At our scheduled time, the person who showed up claimed that she was the in-charge of the centre. We explained to her what happened and she said that she will get back to us on the next day.
She did call at in the evening explaining what happened based on the tutor story. The tutor admitted that she said SHUT UP to my son because he called out the answer BUT she said that she just put her hand in front of my son's face without touching him. This is UNBELIEVABLE, isn't it? What is the intention of she putting her hand in front of my son's face? When we asked them this question, they never explained why. They just kept changing the topic.
Anyway, the centre said that they believe their tutor, which means that two of my sons, who are 5 and 6 years old were lying? They were telling the whole story correctly except on the part that the tutor pushed his head? Come on, we are a person with the common sense here, will we buy the tutor's version?
We are not satisfy with the explanation and asked for another meeting with the tutor but that was never happen. They said that they can't reach her. Let me put it in this way, if the tutor really didn't push my son's head, why she doesn't have any guts to meet us and clarify the whole thing? Doesnt she want to clear up her name?
We asked for a refund for the rest of lessons because we don't feel safe to send our sons to that center anymore and my sons also refuse to go because they are so scare of that horrible experience. They did refund the money to us.
But, I don't know... as a mother, I just don't feel right... I feel like I have to share this with other mums... I believe in both of my sons! Both of them are telling the same thing. And please note that my sons have a lot of activities after school, i.e mandarin lesson, swimming, drawing, piano, taekwondo, tennis etc since they were very young... but they never complaint such thing before. THIS IS THE VERY FIRST TIME (I do hope that this will never happen again) and ONLY WITH THE STEP AHEAD, DISCOVERY PARK, Tsuen Wan.
I really have no idea what should I do next... Anybody have any suggestions?

Have you considered making a report to the police?

Your description sounds like it was a physical assault.

I know.... but we don't have the evidence.

You have your son's word. I think that should be enough to file a complaint. Why don't you call and see?

Were There Any Other Witnesses?