Our approach (we have 3 boys) was to start the kids on easy hikes that they could do part of, then work up to harder, more ambitious hikes. When the kids were very young (2-3) they could do 1-2 miles of a trail like Lake 22 (Mountain Loop Highway) or Rattle Snake Ledge (I-90), and I'd carry them the rest of the way. Once they hit 40-45 lbs then the hikes had to be almost all done under their power - say Colchuck Lake by age 5 (I carried one son up about 500 foot vertical on that). If you work them up this way they'll be able to get up Granite Mountain, Hidden Lake Peaks, Ingalls Lake etc by age 7-9. Mount Saint Helens is not much beyond that.
Good luck!