Car Seats

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I'm hoping some experienced parents can advise. Our older DC will soon be 4 (I need to weigh them to see if they've reached the 18kg limit for their current car seat) and we have a 6 week old baby.
The baby is currently using the maxi cosi Cabriofix and 3 year old uses a Maxi Cosi Pearl or Tobi (one for my car, one for DH's car).

As babies now have to be rear-facing until 15 months and our 3 year old will need to be in a car seat until age 12, we've got some new car seats to consider. The baby will be able to use the Cabriofix until she is a year, but will need something rear facing for a few months more. Ideally, therefore we'd get a seat that can be converted from rear to forward facing. For the older child, we'd want something suitable until age 12.
We've also got to consider grandparents' car!

Any suggestions for what to do for both DC? Ideally we'd like something that isn't a pain to secure if we need to switch between cars, or something reasonable priced so we can afford to buy more than one seat for our older child.

Thanks for reading.

We've just bought the Joie Stages for DS2. It can be used from birth to 7years (I think) and can be installed rear or forward facing. JOIE STAGES

We've been using it for about 3 weeks now and I love it. I actually prefer it to DS1's Maxi Cosi Tobi

Mothercare Madrid is rear and forward facing. It can be used from birth to four and is often £50 in the sales (normally £100).

My five year old has a high backed booster, which starts off with a five point harness which can then be removed so you just use the seat belt, and then the back can be removed so he can use it til 9years old. I can't remember the brand but I know it was about £40.