Get some stainless steel 1/4" thick "triangular-shaped" threaded chain quick-links. Typically rated for over 5000-lbs. The triangular one labeled "S7360" is rated for 39kN or about 8767 lb-F when closed. I used the oval ones, but they bunched up the seatbelts. Not an issue, but not 100% desirable either.
Open link, slip through the seat belt webbing loop where it attaches to the truck at both the latch (side with the red button) and
the point of attachment to the truck for the lower belt.
Use the lateral LATCH belt to attach to the existing seat belt anchors. Puts no more load on the system than it was designed for since it was designed for an adult and the kiddos weight WAY less and therefore have WAY less inertia to restrian in an impact.
Use the over-seat restraint to connect to the D-rind in the rear cargo floor and you're good.
http://www.allproducts.com/metal/kingsnaps/Product-2008411151130-l.jpg