I have trekked at high altitudes in India and Nepal with my children, aged 4½, and 8 ½ year olds. They did not take a piggyback to the base camps but trekked at 9000, 13000 and 19000 ft altitudes themselves along with us and other trekkers en-routes. Taking such small children at high altitudes were extremely risky and against the views of most trekkers and even our family elders.
We do love our children and we are concerned about their safety. It is only a good planning and a careful strategy each time you start your trek which keeps you safe and accident free. In order to ensure almost 100% safety, my wife and I read several books on trekking, before starting out on our first trek in the Nepal Himalayas; unfortunately many vital information were lacking on safety in all these books. I, being cautious of taking children along, decided to jot down points that were necessary for a good and safe trek. This was necessary for us to come back home safe and sound after enjoying over a fortnight of bone breaking arduous journeys in the mountain trails of Indian as well as Nepal Himalayas, with a baby and a child, both of whom walked steadily, keeping pace with adult trekkers in the trail.