I would double check with the car seat laws.. but really if your child has outgrown the 5 point harness then it shouldn't be a problem.. (they are meant to be in a 5 point harness until 4)

I think it is meant to be used with a harness?? or so I thought. I haven't got that far with mine yet. and DD is still tiny so I think she will be in for a while yet.

I have a AHR booster for my son who is 5 and seems easier to use with a harness as I have used a harness in it for a friends DD who was too big for a normal 5 point seat (over 18kg) and it worked really well and easy to get her in and out of it..
hey nic, can i ask you...which AHR booster are you talking about here that your 5yo ds has? thanks