I've never really been a fan of Risk, as I found games from the Axis & Allies family to be a bit more interesting, but I picked up a copy of the Warhammer 40k version of Risk for Nathaniel for Christmas. The boys had never experienced Risk in any form and found it to be a fast and fun gaming experience; meanwhile, I was pleased to see a few new tactical elements in this version, such as unique abilities for faction commanders and major and minor objectives that confer useful rewards upon completion.
We recruited Amy for a full family game today, and it took up a good chunk of the afternoon. Nathaniel played as the Ultramarines (big surprise there!), Matthew led the Chaos Space Marines (the orange minis in the image below), I was the Orks (green), and Amy was the Genestealer Cults faction (purple).
Matthew and Nathaniel each conquered continents early on, giving them significant advantages in reinforcements. I held a continent in the center of the map for a good chunk of time, but ended up taking hits from all directions, while Amy's poor Genestealers were gradually pushed back into the hinterlands. It ended up being a back-and-forth conflict between the boys, with Nathaniel consistently having the upper hand yet unable to deliver the knockout blow.
Finally, it came down to a battle of two single units, one Ultramarines and one Chaos. If Matthew prevailed, he'd get massive reinforcements on his next turn and would have a great opportunity to push deep into Nathaniel's territory. If Nathaniel prevailed, he'd earn his final objective and win outright. After Matthew dragged out the moment for maximum drama (admittedly, with a great deal of help from me), they made their rolls and Nathaniel vanquished the heretics to win the game.
Above is a photo of the map from the final turn. At the top, you can see Amy pointing to her one remaining unit of Genestealers, who survived the war and may yet live on to infect civilizations elsewhere in the galaxy...