Try placing a towel or some foam under the Sleep Guardian to dampen any sound.
If the vibrations are fully waking up your child or if your child is scared of the vibrations, you can reduce their intensity. Access the vibration settings in the app. Select the menu icon on the top left, select User Settings, and then select Adjust Vibration Level.
After changing the intensity of the vibrations, if you don't see your child stir in bed or show any movement for a few nights, you may have reduced the vibrations too much and may need to increase the vibration strength.