Nicolas, you can use the tags down at the bottom of this page to find dog-friendly trails. Keep in mind that dog-friendly and dog-safe are not the same thing.
Generally, the more “wild” the trail, the greater the risks. But even on the suburban trails, there are rattlesnakes, ticks and other hazards. I would recommend keeping your dogs on leash (even in areas where off-leash is permitted).
If you’d like more specific recommendations, let me know what area you’re thinking of. Los Angeles? Orange County?