Adult sized seats and belts in a four seat sand rail that
carries adults and kids. This is not a safe solution. Belts do not hit
child in proper location, and the child could easily slide through the
shoulder belts. If you have a small child (probably less than 3), it is
probably best to figure out a way to strap your child's normal child seat in
and use that. We will look at solutions for children over about 40 lbs.

Crow Enterprises makes a velcro sternum latch for their
shoulder belts. The latch helps keep the shoulder belts in their proper
location so a child won't squeeze through the belts as easily.
PRP Seats and Beard make a simple booster pad that fits very nicely
in the contoured seats. It is 3" - 4" on the bottom and about
2" on the back.

Although my five year old is still a bit small for this
setup, he is much safer now with the pad and sternum latch. And with the
pad, it makes it much easier to pull the lap belt tight.

PRP also makes booster seats that work well with a bench
seat.

Recaro also makes a booster seat that fits well with a 4 or 5 point harness

Recarco booster seat in a sand rail