My wife & I (no kids) rode the Empire Builder over Thanksgiving last year. We rode Chicago to Seattle over the course of 3-4 days. We had a roomette (private room with two beds and a shower). There is so much space on board in the viewing car and the dining car, I would think kids would have enough space to burn off their energy. We had an amazing time and want to do another multi-day trip like that.
Curious to hear about thoughts from other young families. I feel like it would be such a nice adventure to have together.