We had a similar situation with our two children in our 90. A friend built me some beautiful brackets which allowed me to bolt securely two defender centre seats, forward facing, half on the wheelboxes, and added two 3 point seatbelts (from Exmoor) using all the necessary brackets and spreader plates. We used these with two high backed booster seats for several years for a total cost of around 150 pounds when the children were between 2 and 6.
I still have the brackets available F.O.C if they would be of use to you.
Now owning a 110 TDI SW (actually two of them), this demonstrated to me that we really should have bought one a long time ago, so much easier are they to load kids in and out.
The 90 is do-able, but the 110 soooo much easier on the back.