Why "my dog is gentle" is not a barrier
The risk in this setup is not a nasty dog; it is a normal one. Guinea pigs move like prey — low, fast, in sudden darts — because they are prey, and the chase response that movement triggers in a dog is reflex, not character. Dogs who have ignored a cage for months have lunged the first time a pig bolted across open floor. A single paw pinning a pig, or even being seized and dropped unmarked, can break ribs or cause fatal internal injury, and the stress alone of being chased can kill a guinea pig. None of this requires the dog to mean harm, which is exactly why temperament can never be the safety system. The system has to be physical.
Build two layers of separation
Layer one is the pen itself: proper height, grids or panels a dog cannot nose under or knock flat, and hidey houses inside it so the pigs can vanish from view whenever they want — a pig who can hide stays calm, and a calm pig does not bolt. Layer two is distance: a closed door between the dog and the floor-time room, or a fixed gate across the doorway as backup if the pen is in a shared space. Choose the room with the door that actually shuts, and make it the regular floor-time room so the routine is automatic. If your dog can open doors — plenty can — a simple latch or a second gate is not overkill; it is the layer that matters on the day the pen gets climbed or tipped.
Run floor time on a routine the dog learns too
Do the dog a favour and give floor time a shape he can predict: he gets walked or settled with a chew in another room, the door closes, and that is simply what happens at that hour. Dogs who are shut out with nothing to do scratch, whine and fixate, which raises the pigs' stress through the wall; dogs who get their own good thing learn to relax about it. Keep the pigs' session unhurried, end it before they are tired and skittish, and return them to their cage before the dog is let back in, so the two worlds never overlap in the doorway shuffle. Children sit inside the pen rather than carrying pigs around the house — a pig dropped mid-corridor with a dog loose is the accident this whole arrangement exists to prevent.
If the dog fixates or the pigs freeze
Watch both animals' body language over the first weeks. A dog who goes still and stares at the cage, trembles, or cannot be called away is telling you the closed door is essential, not optional, and that pattern will not train away quickly. Pigs who freeze for long stretches, refuse vegetables during floor time or chatter teeth at every household sound are not enjoying their exercise — more hideys, a quieter room and shorter sessions usually fix it. Family Sharing through KinsLe means your chosen relatives can be given access to the pigs' profile.