One of the interesting things I observed with children is that they have a hard time comprehending the time scales involved in space missions. Missions to the Moon take a week or two. "A roundtrip to Mars using current rocket power technology could take several years." (CNN) The (successful) Stardust mission took seven years to travel to a comet and return material. The Voyager missions were launched in 1977, arrived at Jupiter two years later, and took until 2005 to reach the edge of the solar system. And the New Horizons mission to Pluto, if launched this year would take between nine and fourteen years to arrive. (Today's 4th graders will be between 18 and 23 years old.)
However, the children were better able to understand the concept that we couldn't yet bring the astronauts back from most places that'd be fun to visit. It's a one-way trip, folks! All missions we currently do use a heck of a lot of fuel to get off the Earth, then little bits of more fuel to fine tune its path. Most of how a ship travels out to the solar system is using good timing at launch, and then help from the gravity of the planets (called gravitational assists or slingshots). A round-trip mission would have to carry at least three times as much fuel - a lot to launch, a lot to turn around, and a lot to land safely back on Earth again. Additionally, if we have to launch extra fuel for the turn-around and landing phases of the mission, the take-off phase will need even more fuel to successfully get the other fuel off the ground. Not to mention that by the time we got a spaceship to Pluto, 9+ years from now, the other planets would no longer be in the same spots to give us those handy-dandy gravitational bonuses on the return trip.
So, I'm sorry kids, no we can't yet send a person to stand on Pluto's outer moons. It might be possible by the time you're old enough to train for it, if you're willing to give up 25 years of your life in a round trip. If it's not possible yet by then, join NASA and make it possible!