pirates!
Once in a while a roll needs to have pirates come up. The players can then try to run away (if their ship is faster (which opens the door to being able to buy a faster ship later or engines), or they shoot it out with the pirates.
If the kids win the pirates are adrift in space and the the kids get a reward from the locals. If the pirates win the kids lose their cargo.
To keep it simple and visual make a deck of ship cards with 3 or 4 basic size ships for the players, and a card for each kind of "upgrade" with a value in size for each to see if that upgrade can fit in the ship or not. Each size ship comes with a basic layout (1G, J-1, Cargo 10, 1 Passenger Stateroom) and a number of slots leftover for extras to be added later as the players earn them.
Things like faster M-drives to get away from pirates. A laser to shoot pirates with if they can't get away. A better J-Drive so they go to planets on the board they couldn't before. Passenger Staterooms so they can carry passengers and make more money than just cargo hauling.
For cargo there can be animals, food, medicine, whatever. Just make them with fun pictures and a number of spaces each cargo type takes up in the cargo hold. Players pick at random and if they fit then great, if not, then maybe they need to look for passengers (if they have the room), or they can always haul mail ("Aw, Dad! Mail again?!).
Throw in some unexplored planets out there for them to go to and have those add some new cards to the deck with new animals and artifacts for them to get rich with and and have using against the pirates.
Just a few thoughts, anyway.
Or you could have the whole board unexplored and each player gets a scout to start with. Then, with all the mechanics above the game ends when all the planets are "discovered" and the captain with the most credits wins.