You have to be careful you don't fall foul of the new seat belt law for kids. If they are under 12 years old and under 135cm tall then they have to use an approved child seat/booster, which can't be fitted to side bench seats, only forward facing seats. Have you considered forward facing fold up rear seats? They're really expensive new, but I got a couple off ebay. These fit on the wheel arch and fold up when not in use. You can also fit 3 or 4 point inertia reel seat belts to use with these, which would be a lot safer for the kid, especially if you're venturing further afield.