But yes completely judging the Mum as when is it actually acceptable to knowingly let your child do that to other kids?
One thing to consider... had the mother been watching and allowing it "knowingly" or was she far away and "oblivious" of what her child was actually doing? You've said both but they can't both be true, or not at the same time anyway. You did ask what you said that made me call you prejudiced, well another sign of prejudice is trying to have it both ways and arguing two contradictory things at once.
I do believe your report of what happened, what her child was doing, and that no-one spoke to the mother. But all the rest is either supposition or contradictory. Some parents know their child needs to be kept a close eye on, others don't realise until after trouble happens a few times, diagnosed SN or not. We'll never know the truth of that. But you could at least straighten out in your own mind whether you are sure she had seen any of what her child was doing or not.
People who try to avoid confrontation can say and do the very things that confirm their fears, and fail to do the things that challenge their fears. If you'd spoken to her she might even have proved your fears wrong. There is a huge difference in the reaction you're likely to get between "your little boy has hurt the big boy who is playing with him and he's throwing big bricks around, it's not safe, can you deal" and "you are a lazy parent using a lanyard as an excuse". The second is extremely confrontational and so it brought exactly the response you fear. You can't use that to retroactively justify your fears about saying something quite different!
A museum that runs children's events will have policies for what to do if a child is hurting or endangering other children. If necessary they would ask her to remove her child, in the worst case they would probably tell all the other parents to take their kids out and then get her to collect hers. Disability, lanyards etc are irrelevant to that. It's just your own assumption that because he has a disability then no-one will be allowed do anything about it. In truth you sound more worried about what the mother might have said to you than what might happen to the other children.
So have a think about it - just who has been making excuses for doing nothing?