I only wondered because there are 'recommendations' now which suggest a child under a certain age or height should not be in the front. But what if here are no rear seats?
The height/weight and booster seat laws are a load of rubbish as well. The child's weight and height have nothing whatsoever to do with locking the inertia reel belt.
I cannot see how a cheap chunk of injection moulded plastic can improve upon anti-submarine seats, twin side impact bars, front, side and head airbags, seat belts with force limiting pre-tensioners etc etc. If anything the pre-tensioner is likely to yank the booster cushion from under the child and propel him/her forwards toward the windscreen.
The only consideration should be that the top of the belt fits across the shoulder, not across the neck, of the child. This depends entirely upon the adjustability of seat and seatbelt positions.