Why saying उनसे दादा धीरे चलते है is not acceptable?
The subject (दादा in this case) usually comes first in Hindi sentences.
That said, saying उनसे दादा धीरे चलते है is not wrong, especially in spoken Hindi, and it is used to shift emphasis.
Thanks again Vinay92 for your explanation.
"Grandfather walks slower than them," is not standard English. Please correct.