I want to make a particle simulation, however, is there any way to set a sphere as a child and then set an "influence radius" around each particle as like a collision box? I know you can set them to rigid bodies but that really slows down the simulation.
You can add a force to the particles to effectively repel each other as in this answer https://blender.stackexchange.com/a/84426/29586. It isn’t ideal as it relies on the particles being all the same size and it isn’t physically accurate (but is a quick and dirty method).
Another option is to use the Molecular Script add-on (http://pyroevil.com/molecular-script-docs/) which physically simulates the particle collisions.