Parallel bar dips are going to isolate your triceps but one should be weary of the fact that they are also calling in some chest and shoulders when doing this movement. It is easy to increase in weight when you get to the point that your own bodyweight is just too light for the sets and reps you are doing.
1) Hold the bar by both the hands.
2) Neither effective strength and agility exercise is to start with straight arms above the bars and "walk" with the hands down the length of the bars.
If You have medical condition which might need special attention then seek medical or experts advice. Protection from adult is required when children and elder are using it.