Like other action oriented kids shows, the violence of the show varies considerably.
Martial arts fighting from various characters. quite heavy, but okay for kids. MUCH heavier than the 1987 series.The series' action is also much more realistic than the 1987 series.
Also in an early episode The turtles (leonardo especially) are beaten close to death by the foot clan and shredder. Heavy,yet tolerable.
Half-way through the show, All but one of the turtles are murdered in an alternate reality. No blood, but leononardo is stabbed.