I used a Pamlico 160 tandem for 5yrs yaking fishing. It was a barge. It was a sit in yak. I used to move the seat forward when I went out alone.
It was very good on fresh water lakes/ponds. Two adults could sit in it comfortably. The boat is not build for speed so you will have to paddle a bit harder when loaded-especially with an adult up front. With a child, probably not.
For ocean use, with two adults paddling and understanding how the forward paddler sets the pace and understands they are the eyes for the boat because the rear paddler can not see directly what is in front, you would be fine. A sit on top yak would probably be better.
I learned on the SoCo beaches in RI in that tandem when I started. If the boat is back heavy, you might encounter some tricky situations. I say this because with the more weigh in the back, the boat will ride lower in the back, the bow will be lighter and to be honest most tandems done really cut the waves well, they plow through them. There were times even with an adult, I have buried the bow in a wave with the rudder out of the water....if the child is just riding and not paddling, you will have to compensate a great deal more to keep the boat heading in the right direction with any sort of a chop...
With a tandem I always felt the boat was a yak built for two people, who together paddle like one...thats just me.
Please feel free to hit me up with more questions or comments about your yak decisions. I will try to offer any help I can.