What evidence does Heck Tate find to suggest that Mr. Ewell was trying to kill Atticus’s children rather than just scare them?

First, Bob Ewell had a knife when he attacked the children.

“Mr. Finch,” Mr. Tate said stolidly, “Bob Ewell fell on his knife. He killed himself.”

Second, Scout's costume was damaged by the knife, meaning Ewell had attacked her, and the costume had protected her.

Atticus fetched the remains of my costume. Mr. Tate turned it over and bent it around to get an idea of its former shape. “This thing probably saved her life,” he said. “Look.” He pointed with a long forefinger. A shiny clean line stood out on the dull wire.
“Bob Ewell meant business,” Mr. Tate muttered.