For MVR, I was out about 10 weeks, but worked from home after about week 5. Too soon. I had to re-do some stuff. Had memory problems.
I have an office job. I had trouble concentrating and my back hurt sitting at the computer. This time I'm not planning on working from home until about Week 6, then returning to work full time around week 8-10 depending upon how I feel, even though this surgery has been much easier on me.
I find the pain meds interfore with concentration, and the back, etc. is just going to take the same amount of time to heal. I have a 40 minute drive each way, and my back and sternum will take a hit from that, hence the work from home for a while.
With 3 year olds, you are looking at lifting whether you want to or not. I'd say closer to the full 12 weeks to get the sternum to heal.