This was enough for lodging, transportation, and food. It was harder to fit activities into this budget.
We used money given to us by family members for Christmas to take a 3 day rafting trip on the Kali Gandaki River (186USD/person) and to go paragliding in Pokhara (70USD/person). We opted out of a short trek in the Annapurnas because of the cost of permits and trekking guide.
$100 a day was sufficient for visits to Lumbini, Pokhara, Kathmandu, Nagarkot, and Bhaktaphur. In addition to food, lodging, and transportation, this amount covered the entrance fees to Durbar Square in Kathmandu and Bhaktaphur.
So while it is possible for a family of 5 to travel in Nepal on 100 USD per day, if you want to participate in outdoor activities, especially trekking, I would recommend planning on a higher budget.