DD is 9 months, and I found a Stokke chair at the thrift (for $10!!). It doesnt' have the safety harness or any other attachements, like the baby set. So for now, DD can't sit in it. Do I need to buy both the baby set and the harness? Can I just buy one or the other? If I can just get one, which one should I get? Do you use one longer than the other?