iCloud storage is not on a device. Its on an Apple server and can be accessed by any device logged in to the Apple ID that has the storage.
If your computer is using the same Apple ID as the iPhones it should be able to access the iCloud Storage directly.
If your computer is using a different Apple ID than the iPhones, then you can use Family Sharing to share the iCloud storage.
Link-> Share iCloud storage with your family - Apple Support
If what is full, is the local storage on the computer, then then only way to make space is to move stuff off of the internal drive and onto an external drive.