We live in a wood-frame house from the 1880s. The tenant's apartment is just above us.

We rarely hear adults walking above, but their toddler children sound like elephants. How does one explain that little children's feet make more noise than adults'?

It's the gait pattern. Some adults walk in a way that minimizes the impact to their legs when their heel hits the floor. I'd guess the adults upstairs walk in this way.

Toddlers are doing all they can just to stay upright on their feet, so they don't control the impact of foot to floor.

It's the same in hotels. Sometimes they people above you sound like raging rhinoceros when they walk, sometimes you don't even hear them.

The composition of shoe heel and sole might also be a factor in the loudness of sound transmission through the floor. Maybe the adults prefer to wear soft soled shoes but the kids are wearing hard soled shoes?

Good points. Another thing is that toddlers don't actually WALK. Most of the time, they RUN everywhere, which of course means a greater impact (if the adults ran, that would be very loud too). But there's still the great difference in body weight.