what do you do about carseats?

I have 3 children, age 10 months-5 years.  I can't take carseats with me when we fly to California (planning to go end of this year, but might have to fly back with at least the baby for a funeral much sooner).  I don't think my British carseats would be legal in California, anyway.  What do you guys do when you fly back, take your own?  Get them with rental cars? (I heard that most rental carseats are awful, many companies don't even offer them(?)).

My folks have 2 seats which I think will do, and I think I will be sending my step-mum to Babies-R-Us to buy one more,but I am in a panic about making sure that it's ok for my kids to use whichever seats we have or might buy.  Are my children heavy enough, or they tall enough?  My older 2 children are quite small for age which makes it tricky to get the size right.

How do you guys figure all this out?

Im pretty sure rental car agencies have them but Im curious why your UK carseat wouldnt be legal in the USA? I havent ever heard that before. I have four sons and have used my UK seat in the USA a few times and never gave it a second thought. I thought a carseat was a carseat. Please share any info you may have on this subject!!

I've heard that before - but I think it's more that the seats are made to slightly different specifications, rather than a legal requirement.  Someone else posted about this recently, I'll see if I can find their post.

Here it is:

http://talk.uk-yankee.com/index.php?topic=11047.0

I also have 3 kids under 5, and when we travel we hire carseats from the car hire places. We've not had a problem with it yet in either the US or Europe. It beats lugging 3 car seats to the airport. The seats that we have hired were not as plush as the kids' usual seats, but they have all been brand new, clean and sturdy. In Europe the brand we have hired has always been Britax. Hope your trip goes smoothly.