Be consistent and do it first thing in the morning~there typically aren't too many things affecting your schedule when everyone else is still asleep! Applaud yourself every single time you are workout~after all, you are a working mom with two kids and just finding time to take a shower can be a challenge sometimes!
I also find it hard to make the time to exercise because I've set unrealistic goals for myself like 5 times a week in the afternoon (as a mom I'm sure you can imagine the challenges that come up!). After a year of watching that not work for myself, my new resolution is this: a minimum of 4 times a week, first thing in the morning, even if it's only for 30 minutes (because that's more than I was doing 4 months ago!). Baby steps in the right direction are still steps.
With regards to the yoga, if you find it hard to make it to the class, I recommend ordering a DVD for at home from Power Yoga with Bryan Kest. My friends and I used to go attend his sessions in Santa Monica years ago and he's all about "respecting your limits" and not pushing yourself too hard. I can pop in the DVD every once in a while for an hour and my 6 year old will even join in (there was a time when he requested to do yoga with me!).
Hope this helps!