I currently have my 6yr old and 3 yr old in sunshine kids seats and we are expecting a baby in May who will be in a chicco key fit 30 in the middle seat position. My question is mainly for my 6yr old who is almost 70 lbs and fitting pretty snug in his seat. I am wanting to put him in a high back 5 point booster but I'm not sure what would be a good fit for our vehicle and his safety.