Add the berries, rhubarb, and juice to a medium saucepan set over medium heat. Bring to a simmer and cover.
Cook for 15-20 minutes, stirring occasionally until the fruit is very soft. If it starts to bubble vigorously, reduce the heat slightly and/or put the lid on slightly askew to allow some steam to escape. Remove from heat.
Stir in chia seeds. Cover and let sit for about 20 minutes.
Blend in a traditional blender or with an immersion blender. This step is optional but I find that my kids like this better when the rhubarb is blended in.
Pour into half-pint mason jars, freezer containers, or other airtight storage containers. Seal and let cool.
Store in the fridge for up to 5 days or in the freezer for up to 3 months.
Stir the jam 3-4 times while it's cooking to ensure that the fruit cooks evenly.
Reduce the heat slightly if it's boiling and/or put the lid on slightly off center so it's mostly covered but some steam can escape.
Let the jam sit once the chia seeds are added so they have a chance to plump up.