My parents run a pub and we called in briefly to see them last week. DS (almost three) was walking about saying hello to people he recognised. Not running, not making a noise, just walking around quietly looking for people he knows and saying hello to them. One of the regulars had a visitor with him who overheard me saying to my Mum that DS was tired and would need to go to bed early that night (I was due to work, she was babysitting).
"Oh," he said. "That explains why he's behaving so badly. It's obviously okay to bring children into licensed premises and let them be naughty just as long as they are tired. It's against the law you know! Shouldn't even be in here."
I realise now that I should have said "Shut up and fuck off you old git" but sadly I didn't realise that and instead of a foul mouthed rant I did this 
I will admit though, I was thinking "stupid arse" as I at him.
I will give you one thing OP. If someone bigger than you shouts at you without warning it can feel threatening, and a toddler probably wouldn't realise that an elderly man in a wheelchair was not a threat to him.
But you still should be able to realise that, and you no doubt made the elderly, disabled man feel threatened in exactly the same way you are complaining about. Worse in fact, because you had your husband to back you up and because you were swearing and because he was trapped without his carer and had nobody to look after him.
DH's Grandmother was diagnosed with Alzheimer's just a few weeks before she died, but after the diagnosis it became very clear that she had been suffering from it for a long time. She started to say odd, random things for no reason (such as "I don't know why X got so fat, she used to be very slim" when we were not talking about X or her weight and X was nowhere in sight or calling her daughter "that woman" rather than by her name) She also worried sometimes and would tell us "I don't know why I said that, I meant to say this instead but it came out wrong."
That could have happened here. The old man could have intended to say "No need to cry" and had it come out as "shut up boy." Or he could have meant exactly what he said, IDK. But either way, you reacted badly and you were in the wrong to say what you did to a vulnerable, elderly man when you could have dealt with things much differently and still made your point.