I played High Frontier, with the expansion, a few weeks ago. While it LOOKS complicated, it is actually a pretty straightforward exploration game where you buy and assemble rockets, get them off Earth, and explore and colonize (mostly for mining purposes) the solar system. The base game consists of everything up to the asteroid belt and makes rocket building relatively simple. The heavier the rocket, the more fuel (water) you need to move around. Water is the currency of the game as well. The expansion, which boggles my mind that it isn't just rolled into the base game, doubles the board size and expands the range out to the gas giants and adds a lot of additional variables such as solar cycles, orbiting comets, politics, and additional considerations when building rockets. I strongly recommend getting them both together.
The base game should be suitable for children who can manage any other "pick up and deliver" style game, particularly if a more experienced player can assist with calculating burns and managing the tech auctions. Kids will enjoy exploring planets and asteriods, placing mining fatories, and building their rockets from the cards they get. The advanced game has a lot to manage though, but older kids should be fine with it. Dealing with the possibility of war, the more complex rocket designs, radiation exposure and the planning necessary to reach the gas giants adds more paths to victory and needed gameplay depth.
What makes this game really interesting is the level of flavor text. Each card has a couple of stats for game play use, the rest is a scientific display showing the technology and how it works. The rule book is filled with footnotes and the last half or so is just scientific text. So if you want a fun way to introduce rocket science, orbital mechanics, and the cool non-planet things in our solar system, check this game out. Though it doesn't really need anymore complexity, the rocket building is focused almost exclusively on propulsion, power generation, and fuel consumption, only lip service is given to navigation and crew habitats. I wouldn't mind seeing an expansion (or another game) of this depth that dealt with space stations and human survivability in deep space.