Found this info as i'm the same with Soph, she's almost 9kg but only 6 months, here's what it says hun -
When can I move my baby out of her infant carrier and into a forward-facing seat?
By the time your child reaches the weight needed to move to the next stage seat, she should be able to sit up on the floor, unaided, for at least half an hour. When your baby can do this, she is strong enough to turn to face the direction of travel, but do bear in mind that travelling facing the rear of the car is much more protective than facing forward.
Most infant carriers are now tested to 13kg (29lb). My advice is to keep your baby in a rear-facing seat for as long as possible, bearing in mind that your baby's head should not stick out over the top of the seat and if she is in a group 0+ seat, her weight should not be more than 13 kg.
Forward-facing seats are only designed for babies who weigh at least 9 kg, usually at around nine to twelve months of age. Be guided by your baby's weight rather than her age. If your child is nine months old but only weighs 8.5 kg, she should definitely stay in an infant carrier until she weighs enough to move to the next stage seat.