Considering that the end result was him watching his son die...no, it wasn't. The kindest possible thing to do would have been to let him go in and let him die.
Police have their orders. No one should do an act that would harm themselves or others. They on the scene were likely accurate in their assessment that he was on a suicide mission and were bound by duty to stop him by any means available. They did just that and are blameless. They have training to avoid letting passion be the author of another human being's downfall. They stuck with the training, even though they were likely every bit as torn as the father was by the child's terror.
The father was blinded to danger by his love for his child. They prevented a second certain death by his going into the ongoing holocaust they were witnessing in the form of a fire. They're good cops. VERY good cops. I do understand your feelings, though, and you only heard the details and your big heart went out to that father whose heart was rent to think of the horrible end his child was experiencing. The cops wouldn't let two die when one's death was inevitable. We can only trust the judgment they made when reason was beyond the man who was pursuing a certain death for himself rather than to hear his child suffer. I pray for all of them who mourn the child who was indubitably beyond help.