Hi
We took our daughter on the China trip when she was 5 - there was also a 7 year old. She had done a lot of international travel, so we knew how she coped with jet lag etc. I also talked with
ABD about it was min age 4, suggested age 8 - and decided she would be OK. The trip was amazing, but exhausting for all - adults included! Our guides were nervous to have a 5 year old, but none of their fears came true!
We went to South Africa when she was 6. That one, only when we got to the game lodge did we realize that 6 is the minimum for game drives, and the driver can ask you not to return....our driver was great, and helped her realize she needed to be quiet and patient.
We are waiting a year or two to do Costa Rica and Galapagos though as they are more physically tiring from what I have heard. And Peru too (but for that one I prefer the itinerary offered by other companies.....)
So yes, little kids can do it. There is another thread that talks about the tradeoffs, and others describe those much more eloquently than I can.
Have fun!