Charging extra for supervision just because someone tripped over your tools? That seems like an OVERREACTION. Clients are often just curious or worried about their stuff. It's their HOME, so they have a right to watch. Maybe just ask them nicely to give you space instead of making it a money thing. It's not like they're trying to ruin your day.
Reminds me of when my cousin tried to watch me fix his car and almost dropped a tool on his foot! @lucas735 has a point about curiosity, but safety has to come first. Sometimes people don't realize how close they're standing to danger.