Hello Popi!
Many things are complicated for autistic children, especially communication. I saw an example of that last week, when a mother of a three year old boy wanted him to greet his speech therapist. "Hello Popi!" his mother told him to say. The boy said: "Hello Konstantinos". The mother corrected him, saying "no, Popi is there. You must say 'hello Popi!" And then we all laughed a bit with pity that the poor boy does not understand.
I consider it a big mistake to insist in having children to say something correctly all the time. They will NOT learn it that way, but they will only get more confused. One cannot learn mathematics on a university level, if one doesn't know multιplication yet! Since they make this 'mistake' (which in their eyes is not a mistake, but very logical, I am sure!) we have to understand that they have not obtained enough reciprocity yet to be able to understand how we greet each other. First they will have to become more social, then they will know how it works.
Correcting all the time -of course trying the best we can to help them understand- we only succeed in confusing them more and on top of that we give them the message that it is not good what they do. And how many times a day do they go through this procedure?? This is not the way.
Playing with them though, giving them socialibility to experience, simplifying communication in order to make it understandable for the children, that is what is needed to have a chance to succeed. You will understand it by yourselves, when they greet well some day, without you interfering.
Besides, why not be happy with a simple 'hello', without a name, so that the children greet correctly!