I have Britax Evolva Ultima which is suitable from 9 months to 11 years. My 2.4 year old is getting increasingly uncomfortable in the integral harness, particularly the buckle which has wide pieces which are now digging into her legs. Its all adjusted to the maximum it can be as far as I can see. She is big for her age and it says in the instructions that a seat belt can be used instead of the integral harness from 15kg. She is nearly 16kg but obviously is still very young so what should I do?