hm. a machine solves both problems; I mean, I don't have kids but most of the cardio I've done in the last 5 years or so has been on a machine; instead of watching children, I watch silly action movies. For me? it's to distract me from the monotony. Action movies were made for my attention span; but it seems you could just as easily supervise children.

That said, I certainly feel a lot more difference from resistance training than from cardio, and diet changes have been more effective than either in controlling my weight. (e.g. an hour a day of running is not going to keep me from ballooning out if I use mountain dew as my primary caffeine delivery mechanism.)

I mean, you can get a really nice treadmill for a grand and a half; by the standards of medical care, that's really cheap; Personally, I've also had some success using the manual type (which can be had for $50 or so on craigslist) unlike most other cheap cardio equipment (I utterly destroyed a rowing machine through a month of daily use... most low-end cardio equipment wasn't designed for actual use by a 200lb man.) the manual treadmills seem to be fairly durable. "I'm going to go push the shopping cart" I'd say. It does take a bit more concentration than an automatic, just 'cause if you stop, it stops, but eh, it's certainly usable, and you get a little bit of upper-body work done, too; it really is very much like pushing a heavy shopping cart.