I have to say that I also have a child that runs off and we trust very few people with her for that reason. We have had the same situation with older relatives not really accepting that the kids are different and we feel that they cannot be trusted with our children if they do not fully agree that they are ASD and have issues other children do not have.

When these people ask to take the children out we politely say that you are older and the kids have a lot of energy and they are hard to handle even for us and we do not want to burden them with not being able to keep up and being exhausted by the end. Generally this had worked and they do not ask anymore.

To me this has nothing to do with us as parents letting our children have a life without us and all to do with safety. Honestly if they get offended and think we are being overbearing I could care less. After being on this journey for so long I have learned that I do what is best and everyone else can either deal with it or leave

Not knowing your mother, its hard to know exactly what all background here is but do you think this episode could cause her to be more diligent? You are really lucky that you have family who is around and actually wants to spend time with your children. My dad moved really far away and my inlaws visit a lot but they don't ever want to do anything with the kids.
I'm not at all minimizing what happened, that would be terrifying but maybe it gave your mom a dose of reality. Just a thought...

My mom is the same way and I don't let her take my son out on his own. He doesn't do it much but that makes it all the more dangerous because we'll get comfortable and forget about him running off and then all of a sudden for no reason he will dart off in to a street or something. Luckily as he's getting older the occurances of this happening are less and less and almost non existent now. I don't think my mom is capable of changing her behavior when it comes to things like this so I don't let it happen and try to avoid conversations that I knew will end up in hurt feelings and arguments as best as I can. I guess the question comes down to what BrianBass mentions... can your mom *learn* to handle the situation? If yes, then give it a go. Maybe tail her at the movies, sit near the exit to ease your anxiety and be the net in case someone decides to run.

Thank you all for your replies.

Judging from what you said it sounds to me like she has learned her lesson from your reaction to the situation, especially since she was on the phone with 911. It sounds like she now wants to prove to you that she is responsible and knows the consequences. However as his mom it also sounds like your boundaries are being tested and your will as well. Can you teach your mom how to handle him in public? Like give her the invisible boundary in feet that you use so you can catch him if he starts to bolt. You could even have a practice session to increase her reaction time. That could give you peace of mind. I'd miss it if my MIL didn't take the kiddo for a night every now and then.