Peel the mangoes and cut the fruit away from the pits. Place the mango flesh and lemon juice in a blender and blend until completely smooth. Line 2 large baking sheets or 4 dehydrator trays with parchment paper or silicone mats. Pour the 1-2 cups of mango puree onto the prepared trays and spread it into a thin, even layer, making sure there are no thin patches (save some of the mango puree in the blender).
Something you can do is pour the puree in two long yellow strips across the tray, and then two pink strawberry stripes beside which makes it look cool later.
Remove the stems from the strawberries. Then with about 1 cup of mango puree left in the blender, add the strawberries and blend until the mixture is very smooth and thin. Pour the pink strawberry mixture onto the trays in lines beside or over the mango puree. Sprinkle the chia seeds on top of the puree fruit if using. Use a spoon or skewer to gently swirl the mixtures together for a swirl effect.
For the dehydrator method, dry at 125°F for about 5-8 hours. For the oven method, bake at the lowest temperature setting, about 170°F, for approximately 6 hours. The fruit leather is ready when it feels dry and even spongy to the touch. It will peel away easily without sticking.
It will depend on how thick the fruit is spread and if you have a fan in your oven or dehydrator on how long the process will take.
Roll the parchment paper and fruit leather into a log and then cut into about 1.5-2 inch chunks. Keep them in an air tight bag or container and they will last for months.
Notes
How long does fruit leather last? I have fed my kids fruit leather for months. The key is to make sure there are no wet spots after dehydrating it because that's what will go bad. And what you need to do is see it in a ziplock bag.To store them for an extended time, add a paper towel into the bag or container you keep the fruit leather in to absorb the moisture.If your fruit leather became too crispy or over dried in some areas you can sprinkle or spray it with water to rehydrate the leather and make it softer! Often, you will need to do this on the edges.