The maximum permissible weight for our shield is 18kg, it will state this somewhere on the product. If the child were under 18kg then they can continue to use the shield, normally there should be plenty of room for children under 18kg as the shield can come almost all the way out of the guides.
Also we should consider the position of the child’s head in relation to the headrest, as the child is growing their eye/ears need to be approximately central into the headrest, raise the headrest up or down accordingly and when in grp 2,3 mode (booster stage) we should also be checking that the seat belt sits across the childs shoulder.
Finally once the childs eyes have exceeded the very top of the headrest (in its most extended position) or if the maximum weight of grp 2,3 - 36 kg is exceeded then the child is now ready to use the adult seat belt in the car, but this will not be for years to come.
Chris
Kiddy