Yes we ended up staying in Nendaz and skied both there and Siviez with the family. The village of Nendaz was great, lots to do and really great restaurants. The drive up the mountain is not for those with a fear of heights

For the skiing I think for true beginners as small children the Siviez set up is better. We were with 2 three year olds, a five year old, and a 6 year old. Those stairs in Nendaz with the number of skis we had to carry were not ideal. However once you are there Tracouet is great for the kids. Our five year old had a huge tantrum out of fear in the freezing snowstorm in Tracouet the first day and it was not fun getting her and the skis back up the stairs. The ski instructors needed help getting all the kids and the equipment down the steps. For our adult friend who is also a beginner, Nendaz was perfect and the stairs were not an obstacle.
Siviez was a very easy and close drive to ski with everyone. Parking was not an issue, even during the busy time over the Feb break. All of the children (and adults) were able to keep their same instructors from Neige Adventure who met us in Siviez. It was a great experience, with everyone skiing from the top of the mountain by the end, with our littlest happily playing in the nursery at the bottom of the slopes. The main trail did get very crowded in Siviez however.
It is best to stay in Nendaz, even with skiing in Siviez I think, because there are more restaurants and things. The views from all of Nendaz are amazing.
We never made it to Villars to give any feedback. Recently though, we did end up in Morzine which was fantastic for families with small children. It is a lower altitude though, so not sure what it would be like now.
Thanks for everyone's advice!