A simple resistive divider or attenuator is the way to go.
First, you need to specify your output impedance. Most function generators have an output impedance of 50 Ω. It's a practical impedance for reasonably low noise, and it will keep calibration with any other testgear you have.
As you're only going to 100 Hz, a simple single stage pi attenuator like this is sufficient.
simulate this circuit – Schematic created using CircuitLab
If your function generator is specified to work into 50 Ω, then it's worth having R1. R2 is needed to establish the output impedance. R3 gives you an approximately 1000:1 reduction in output level when the output is loaded with 50 Ω.
For higher frequencies, a large resistor like 25 k is not a good idea, and a multistage attenuator is flatter.
If you don't want to load your function generator, or want a lower or higher output impedance for some reason, then you can scale the resistors appropriately.