Seems aggressive - so what's the youngest reasonable age to take kids that will still be fun and safe? Maybe 8 years old?
I am not a child, nor do I have one, and also I am probably in worse shape than a child of 8, but I hiked the Inca Trail (the less strenuous section) and it was difficult. If you attempt it with a kid around that age, I would take the train in to Aguas Calientes where you can then decide to do the 5-hour walk or take the bus up to the base of the Machu Picchu site. Bonus: The city is built on top of natural hot springs and you can get a day pass to the outdoor hot tubs.