Actually, if the Walmart is part of a plaza, they do not control the parking lot .. case where I live.
My local Walmart does not post No Solisiting signs either.
I would not do what these parents did .. but quite honestly, we don't know all the facts. There may have been other adults very near to this little girl ... she only pointed out where her parents were because that is what she was asked.
Would we hear all this commotion if it had been a girl guide selling cookies?