We did it in 2015 with our then 5 year old daughter. Stayed in the glass igloos in kakslauttanen, which were surprisingly very cosy and proved very useful as we can see when the northern light is visible, then we would tumble out of the igloo to get a better view.
Activity wise, apart from the usual skiing and the Santa village, then their was the snowmobile and husky sleigh safaris. All these can be arranged at the hotel, but we would highly recommend the reindeer sleigh safari. We did the night safari into the woods at night. Amazing view of the stars and sky in the dark woods. These were run by the friendly Sami people, and we stopped in the middle of the forest to build a campfire and where he brewed hot drinks and warm snacks whilst telling stories about the Sami people under the open sky. We had the best sighting of the northern lights here, and its quite an experience where the only noise is the reindeer and the sleigh on snow with the only light coming from the dark sky.