Book a family igloo and dinner in advance, especially if traveling during popular times.
Depending on the snowmobile safari provider selected, there may be age limits on what the kids can do. Check before booking. They will have fun being pulled behind in a sled, but if your kid really wants to drive one, find a provider that has mini snowmobiles for your kid's age. Also, there is a shuttle to the igloo hotel.
In northern Lapland, the aurora borealis shines about every other clear night between September and March. In southern Finland, they are visible about 10-20 nights per year.
You can save money by sharing a snowmobile and swapping drivers halfway through. The igloos are expensive, but worth it.