I went through this dilemma a few years back- how to safely seat two kids in the back of a 90 and maintain useability for carting around loads when needed. Firstly, I completely discarded the notion of side facing seats. probably OK carrying adults on a hunt, off road, but not for kids. Lap belts on side facing seats probably add to the chance of injury, so I discounted them too. I looked at various options of minibus seats with integral three point belts, but wasn't happy with those either. Quite a lot of structural steel would have needed to be added underneath the tub to provide a solid mounting. I ended up biting the bullet and buying a pair of Exmoor forward facing, folding rears which are OK, but not brilliant. (I do have a used Exmoor Trim high backed rear drivers side folding, forward facing seat that I might sell complete with brackets and belt, but I am in Dorset and can't be messing about with couriers). Truth is, it's not cheap to do safely, and safety is never cheap!