Slice and core the peaches and plums. Add them to a food processor and pulse until the fruit has no lumps.
Add in the lemon juice, sugar and water and pulse once more until the mixture is well combined.
Pour the juice into popsicle molds evenly, ensuring that they don't overflow. If using an instant popsicle maker, be sure to insert the sticks before pouring and then wait 15 minutes to freeze before removing. If using traditional molds, place in the freezer for at least four hours before removing from molds.
If using an instant pop maker, remove carefully. If using traditional pop molds, fill a sink with hot water and immerse the mold into the water (but don't have the water touch the lip!) for a few seconds. Then pull them out.
Enjoy! Place in freezer bags and lay flat in the freezer.