Preheat oven to 350 degrees. Cut the flesh off of your clean peaches, working your way around the pit. No need to peel them. Dice. You'll need the pieces between a quarter inch and a third inch square.
Put the peach pieces and the Splenda in a small saucepan. Cook on Medium or Medium Low, stirring often until the peaches are soft, almost mushy. This will take 5 to 7 minutes.
Remove from heat and stir in the mascarpone and the pecan pieces. If you are on a strict low calorie diet or there's a nut allergy in your family, the pecan pieces can be left out. Mascarpone cheese is not low calorie but, when you are spreading a half cup of it across at least 6 servings of wontons, it won't amount to many calories per wonton. Because the peaches are still hot, the mascarpone will melt. Take this mixture and put it in your freezer just for about 10 minutes to get the mixture to thicken back up a bit. You can do this while you are getting your wonton wrappers ready.
Lightly spray your cookie sheet(s). Don't spray on too much butter spray or your wontons will be greasy.
On your work surface place one wonton wrapper. In the middle place a teaspoon of the peach-pecan mixture. Don't try to overfill these or they will be difficult to seal. Moisten the bottom edge of the wonton wrapper with fingers you've dipped in water. Fold over the top edge to make a triangle. Now just press down and pinch the wontons until they are closed, keeping the filling inside. Place on cookie sheet. You can get about 15 wontons on most cookie sheets so you might need to bake two batches or use more than one cookie sheet.
Keep going until you have all the wontons filled. This job can be done by a responsible child who'd like to learn to cook or a handy adult who wants to help. This is a really quick dessert if you don't mind the assembly time.
Bake your wontons in the middle of your oven until they are lightly golden (12 to 14 minutes at 350 degrees). Immediately put them on a pretty platter.