The MF was my step-up from class III trips and I first ran it at 2.5 feet. Great level for an intermediate rafter the first time. However, I was pretty well exhausted the first two days from the constant concentration that was required. I was lucky enough to be following some very experienced rafters, who kept a close eye on me. I would not take a 5 year old on your first MF trip. We started the "river kid" in our group on the Main Salmon at 4 years old, but didn't take her on the MF until she was about 11-12. If you are trying to split your attention between the river and the kid, something will suffer. Just my 2 cents.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb