Types of seating fit into rough age groups, but it's more important to make sure your child has correct equipment for their size. At the moment, this means a rear-facing seat for Brookland but she's getting near the stage when she'll outgrow it, as these seats are only ok for children under around 29lbs.
Rear-facing seats are designed to support babies who can't sit up on their own and can be used in the front as long as there's no airbag. But once you can see your child's head over the top of the seat, it's time to move on to the forward-facing seat. Austin has one of these and she's 3.5 years old, but again, you need to go by weight: around 20-40lbs is fine for these seats.
Child seats must be fitted correctly in order to work, so when you head out to the stores, try and test the products out in your car and think about any other cars that your child needs to travel in as well. A good retailer will suggest different types and help you fit the seat yourself, but you can also find videos on the web.
Many newer cars come with a system called Isofix (also called LATCH), which means there are points built into the car which match corresponding car seats. There's less risk of incorrect fitting with this system, but we've also used seatbelt systems in our car and they're no less effective so long as you install them properly. It's good to look at both options, especially as Isofix seats tend to be more expensive.
Lastly, always buy new, rather than used. Seats from a trusted friend or family member are ok if you know the history, but a second-hand seat from elsewhere could have been involved in a crash and have structural problems. I found as many brand reviews from other moms as I could online before going anywhere; it's best to be as well-informed as possible when it comes to child safety in the car.