First of all, I have to say, YOU GO GIRL! Taking care of yourself is so important for YOU and your family, although I understand how easy it is to feel guilty... You are in a rather safe environment at the gym and if you can keep a relatively good watch on your daughter while you're working out, then go for it!
Your daughter is also learning a valuable lesson very early in life! If she sees you exercising and taking care of yourself, you're leading a GREAT example for her future. Kids are pretty good at entertaining themselves for 30 minutes or so!
If you are sticking to cardio, do 30 minutes and then get your daughter involved with stretching with you after you're finished. If you want to weight train, but worry about the time, do full body exercises like a lunge with a bicep curl or shoulder press- check out SHAPE.com (or buy the magazine) for some ideas and helpful hints to maximize your workouts! :) GOOD LUCK!