The 'Golden Circle' is the trip that cannot be left out in your stay in Iceland! 
It consist of:
a) Thingvellir, the national park where the oldest parliament was founded,
b) Gullfoss one of the most beautiful waterfalls in Iceland,
c) Geysir, the biggest geyser in Iceland
d) Laugarvatn, with natural steambath with steam from the ground,
e) Kerid, a volcanic crater lake
f) Hveragerði, a small greenhouse village and
g) Nesjavellir, the largest geothermal power plant in Iceland.
These spots can all be visited in one trip but it can also be divided into several trips if you like to have more time at each place and take a closer look. Especially for kids it would be good to divide it at least into two trips. Then I would recommend one trip with Þingvellir and Nesjavellir together, as you can really experience those places and the road between Þingvellir and Laugarvatn is maybe not too good, and Laugarvatn, Geysir, Gullfoss, Kerið and Hveragerði in another trip.
