If you use the reroute nodes available in the Material Editor as of UE4.15, there seems to be a bug with Material Functions that causes scalar parameters to not be visible in child Material Instances.
Create a new Material.
Add a ScalarParameter to this material.
Connect this ScalarParam into a Material Function, and then connect the Material Function into one of the final Material Attributes.
Add a Reroute Node between the ScalarParam and the Material Function’s input pin.
Create an instance of this Material.
Note that said ScalarParam will not show up under the parameters for that Material Instance.
If you remove the Reroute Node, the ScalarParam should show up for the Material Instance like normal.