What age should my child be in a car seat?

A child should be in a car seat or using a booster seat until they are about 12 years old or 145cm in height - weight is not a factor when deciding.

My child has to put the seat belt under his/her arm as it's too uncomfortable - is this ok?

No, it's not ok.  If a seat belt is used properly it should fit across the shoulder - if it is too high on the neck then a booster seat should be used to raise the child to make it sit more comfortably.

Do you take second hand car seats?

No, I don't take used car seats. The official line is that car seats have no guarantee that they haven't been in an accident - they quite often don't have the users manual anymore and they also may be out of date in terms of latest safety guidelines. I would far rather people use a secondhand seat than none though so I would suggest you put an ad somewhere to give it away or donate to someone you know.

I sit in the back but don't Buckle Up as I'm an adult - is this ok?

No, it's NOT safe to sit in the back without buckling up.  At a reasonably slow speed collision of 40 kilometres per hour, an unrestrained passenger will travel up to 60 times his or her own bodyweight.  That would be like a bowling ball flying around the inside of the car.  Being in the back does not automatically stop a body from flying out of its seat - the ONLY thing that will stop you is a seatbelt and that includes being in a taxi!