I would agree that long trips on a small boat might not benefit a small child. If you use that argument then you would also need to factor in early life experiences on a child exposed to the wonders of discovery not available in the controlled environment of California middle class life. Everyone chooses they're own path in life along with teaching they're offspring and exposing them to what they, as parents, feel will prepare them for a happy and successful life.
If this trip was an indulgence in one or both parents without regard to the children it would be tragic. To believe in, prepare for and engage in a family life experience which they believe is for the entire families benefit is another. In this case, from they're website it seems the latter.
Endangering your child in many cases is very subjective. I'd say that having a child out of wedlock while an unmarried teenager with a non existent male parent is total child endangerment and in general dooms the child from conception. yet as a society we accept it.
I only have what I've gleaned from a short time looking at they're site. It appears they love life and want to share it with they're children. I hope should they decide to continue that they have success.
The one thing that does cause me to pause is the circumnavigation thing. That does push the envelope a bit on very small children and teaching life experiences. That is a lot of long passages. Continuing with the Mexico cruise might have been the wiser decision.