I'm the mother of 3 girls and I've found the best way to balance family and fitness is to mix the two. How about giving wifey a break from the little darling and taking baby girl on some looonnnggg walks, rides, play time at the park etc etc? What's good for the gander is mighty for the rest of the flock... A good jog stroller is a great investment or one of those baby back packers, especially if you compare it to the cost of a gym membership.
My honey likes to take the girls for a little spin around the park. All of them are old enough to pedal their own bikes now, but he's been doing it since they were all little tiny tots and had to ride in a kiddie seat behind him. Lately they've been rollerblading, which is pretty hilarious, but we've been going through a lot more band aids.... Also you might look around for a pool, little tots love the water and now is a really good time to start swimming lessons. Swimming is a great calorie burner even if you're just goofing off, because your body burns extra cals just trying to stay warm. Our youngest just turned 4 and swims like a fish, but she's been in the water routinely since she was 3 weeks old.
Granted you might not necessarily get the same burn in a workout with kids that you could get on your own, but still it's physical activity and it's raising your children with the right sort of fitness habits.
Starting weight: 244.6 lbs. 10/01/09
Pounds lost: 80.6 lbs
Current weight: 164 lbs
Goal weight: 120 lbs
Weight to go: 44 lbs
Goal Date: 1/1/13