Nearly died of embarrassment yesterday

I have a daughter who is severely Autistic and non -verbal. I am beginning to think she's retarded. I know how that sounds.... but this is what happened yesterday. I am so upset that I haven't been able to eat for two days. We were at a restaurant (the 4 of us and my mom "grandma"). My daughter's Grandma took her to the restroom, but was only holding on to one hand. Glancing over toward them, I noticed that there was a family sitting there with a small baby. I worried that my daughter might touch the baby seat, the blanket or the buckle (fascinated with buckles), as she has so often in the past. I almost got up, and ran over -but decided to just let my Mom (Grandma) handle it. Never could I have dreamed in my worst nightmare what was about to happen. I didn't see it, but my daughter slapped the three month old baby on the head "three times". The parents were very upset (understandably) -but would not accept our apology at all, and yelled at us as they walked out, threatening us with legal action as they walked out. They demanded my name, phone number, etc...., informing me that they were going to take the baby to the doctor. I had the composure somehow to get their phone number -then started crying in front of everybody. My son (also) nearly died of embarrassment, and cried too. All day long I worried that we would be sued (still no guarantee we won't). I was scared to call them, so my Mom did. She said that she was told that they took the baby to Children's hospital, and the baby "screamed the whole way" but that she was "okay". She informed my Mom that if they had to pay a co-pay, we would be expected to pay it. Of course, we agree. I feel like we are like scum compared to normal families because we have a daughter like this who is like a weapon that could go off at any time. I don't plan to ever take her to another restaurant or store, and dread what could happen if I do. She has NEVER hit anyone in a place like this before. She has, however been hitting some of her classmates at school. We don't know WHY, where she's learning it. I don't have any friends here. My mom is mostly supportive, but also critical sometimes. My husbands family is totally unsupportive, with the exception of his Dad -in fact they NEVER EVER call because they don't like me. My husband still calls his "Mom" . I was beyond upset yesterday after he called her and asked for advice about this. Why should she even be told? She could care less if we live or die. My husband is totally in denial about this. My daughter receives ABA (behavior therapy) twice a week. I don't know what they can do if she is retarded though. I feel really hopeless right now.