Add data to Multi-select field

No, there is not a way to filter that.

An alternative is to go to a proper Many to Many table set up where you add multiple child records which then can be rolled up into the parent if you want.

There is an app I put in the exchange called User Focus Examples and that does a demo of how the Quick Select can work to allow users to quickly select the children for a satisfying user experience. An advantage of the quick select method is you launch the user into a report which can be filtered however you like so that the choices offered are relevant 

Or contact me directly for a demo.