I would definitely go to the gateway. The Children's museum and Hansen Planitarium are well worth the short trip. The planitarium has a 30 minute show that is incredible and kids love it. Any of the airport taxis will gladly arrange to pick you up at the gateway, when you are finished, at some pre-determined time; they just don't get that much business in Salt Lake so they are pretty hungry, particularly for airport runs. Good Luck.