Kalum, agreed, the nutserts loked a bit iffy. I used washers and used grade 8 bolts and locknuts, no need for a plate. There are a couple spts that it is tricky to get to, but there are several and more than enough bolts to hold the setup together. As far as a harness, I am using JK rear seatbelts. The safest and best approach is using the kids car seats if you still have them. I have friends that run Radian Diono RXT seats and they are by far some of the best car seats, have aluminum frames and you can fit two very well from babies to tweens. They support kids from 5 to 120 pounds so you are definitely set. If you look for the JPMagaizine or Fourwheeler article catching up with Katie there are some pics and details on the car seat. Actually the picture in the article kinda makes you want to make babies if you do not have yet

JK
Recaro, Corbeau, etc also make car seats and suspension seats for kids.