Whatever you choose to do make sure you stick to it. There are a ton of stories and studies about people wasting money on Gym memberships because they never go. There are also many people that buy tread mills and use them for the first week or two and then just leave it in a corner to collect dust.
You know your kids best. If they are the type to want something new and then not use it a short while after, the same will happen with the treadmill. You'll have to come up with some rules like "whenever you watch tv you have to be on the treadmill". Things like that make people use it!