If you’re going to hire a car, you will need a car seat for any child up to about 7, or any child that is short. You can usually hire one from your car-hire company, but they are often very expensive to hire. When we went to the UK a few years back, we found that it was cheaper to buy one for £20 when we got there, instead of paying £65 to hire one. We did an internet search for the closest baby shop to Heathrow (in Slough, if anyone's interested), then went straight there from the airport. At the end of the trip, we left the car seat in our hotel room with a note on it saying “free to a good home”. We had enquired about donating it to Oxfam, but they weren’t accepting any baby goods.
On our trip to the USA and Canada, we took our own car seats. As our children were a little bit older (though very short) we opted to take half-booster seats, which went into the checked luggage. On a subsequent trip to Europe, we shopped around and found some even smaller in size and weight which easily fitted in our cabin luggage with all our other belongings (we had no check-in luggage).
Check the regulations of the country or countries to which you will be travelling. In large countries, these regulations can change from state to state or province to province, so you will need to check the state’s individual regulations. Places to check regulations include current edition driving atlases or up-to-date travel guides. You may also be able to find the information on the internet. In many cases, other countries’ regulations are more relaxed than Australian ones, meaning you can take whatever suits you. But check if there are specific regulations with which you will need to comply.
If you do decide to hire a carseat, check very carefully that it suits your needs. I have heard horror stories of people hiring cars complete with outdated and unsafe car seats that stink of vomit or are covered in food stains and crumbs. Check as best as you can before you book, what the seat you will be getting is like. If the car-hire company promises anything, keep a copy of that promise in writing. When you pick up the car, check the seat over properly. Make sure you can install it correctly and safely. Insist on reading the instruction manual. If you are not happy with what is being offered, ask for a different seat.
If nothing is suitable, you have a few options. If you are picking up at an airport, don’t sign the car-hire contract, and go to another car-hire company, and see if they can provide for your needs. Or tell the staff that you will take the car seat to the closest baby-goods store, purchase a proper car seat, and drive back with the hire car seat and return it for a full refund of seat hire fee.
When travelling with kids, safety in the car is of paramount importance. Take or hire a decent car seat for each child who needs one, and comply with the local regulations. The driving speed on some motorways in Europe is legally quite high - unrestricted in places like Germany - and frequently exceeded by local drivers. Good car seats are essential.