We did the rafting as a family with two children (10 and 12 years old). The river is not very wild at this time of year (January), as it doesn't rain much, but we were able to enjoy the beautiful nature. There are a few short places where you have to row along, but it is not dangerous at all.
Jonathan was a very good, competent guide and we had a great time with him. The driver was also very nice and they picked us up at the hotel and brought us back at the end.
We would repeat the tour on our next stay in Huatulco and could hence highly recommend it.