I have a family-room circuit with 5 outlets and a one-year-old who thinks it's funny to try to stick things in them. So I thought it would be a good idea to add GFCI protection (and tamper resistance). I know that in theory you only need one GFCI box to protect the whole circuit, the trick is to determine which receptacle is farthest upstream and replace that one, making sure you get line vs. load correct. So my plan was this: take all five boxes out of the wall, disconnect and separate the wires, flip the breaker back on, and walk around with my non-contact voltage tester looking to see which black wire still has voltage on it, implying it is the most upstream. Well, it turned out that when I did this, every single black wire at every single outlet still had voltage. I'm sure this wasn't a false positive because none of the white wires triggered the tester. I take this to mean that there is one main wire that circles the perimeter beneath the floor, and right below each outlet there must be a junction box with a separate wire going up to the receptacle. Is my reasoning correct, both regarding how to find the first receptacle and what it means that the technique didn't work? If so, does it imply that I can't protect this circuit by replacing a single receptacle, and would need to either install a new outlet farther upstream or use a GFCI breaker instead?
I take this to mean that there is one main wire that circles the perimeter beneath the floor, and right below each outlet there must be a junction box with a separate wire going up to the receptacle.
This sounds most likely, however, if done to code, junction boxes shouldn't be hidden in inaccessible locations (walls/floors/ceilings that do not have access panels over the box). If there is a floor beneath the floor or an attic above, this is the most likely location of the junction box.
You may be able to find an outlet further upstream which supplies power to the other outlets, but in comparing the price of a combination GFCI/AFCI breaker (~$40) with a single tamper-resistant GFCI outlet GFCI (~$20), I personally wouldn't spend any more time investigating for the additional $20 and would just swap out the breaker.
I believe that a much simpler answer would be to install safety covers on your outlets. You could likely do all the outlets in your house for the cost of a single GFI outlet. In my experience babies and toddlers lack the grip strength and dexterity to remove these.