Heel rollers, heel wheel skates, or heel wheels, have a fundamental flaw in their design.
If we humans were to pick between standing for a minute on the front section of our feet or the heels of our feet, odds are we would be more comfortable on the front section. The heels generally bear most of our body weight. But if our weight is channeled only to the heels, they seem to tire soon (the calcaneus region). In contrast, the front section of our feet do better at balance control and managing the weight.
Coming to these heel rollers which seem to be popular with kids, they seem to have a flawed design. Each roller has a single pair of wheels, worn over shoes, and positioned towards the heel. This leaves the front section of the feet free, allowing for balance control. Yet, this puts the axis of the wheels in line with our center of gravity, increasing chances of rapid falls backwards. That in turn, increases chance of injury to the head.
What if you wore the wheels over the forward section of your feet, closer to the toes?
While this would remove that balance control that the front section of the feet offer, it would increase chances of a forward fall. Of the two, this one is slightly better as the user has a shot at controlling or dampening the fall. But, now the wheel’s axis is outside of our center of gravity, making the ride unstable and not enjoyable. Unless one skates only using the forward section of the feet. [Imagine a woman in heels, but with wheels under the front. Does that look remotely comfortable?]
Unlike roller skates, heel (or forward) rollers would be a good way to induce a back pain due to its unstable design, The lower back in particular would be working extra hard to counterbalance to constantly maintain or regain balance.
So in all, not the best idea for a fun alternative to walking for kids.