A pony of that type and size should have no problem. But, there are so many variables to the equation. You need to look at the force it takes to start the load and both pull the load (the easy part) and stop the load (hardest part). The old rule that we used for starting drafts that your horse can comfortably pull 1/2 his weight on large wheels, 1/3 his weight on little wheels, and 10% (or less) of his weight on a dead-weight pull. To determine if a horse is overloaded look at that point traction problems start happening, the horse overheats, or the horse refuses. You don't want to get to that point but in the event you do you will know you are past your animal's limit.
Two adults behind a pony could well be an easy load on hard-packed flat ground, but two little kids in the same cart could cause problems if they are in deep sand, loose ground or hills.