What seat for 13kg toddler that will last

[Ilythia]

We have just had to buy a second car and need 'extra' seats for the DD's in case DH needs to drive them anywhere (would only be sporadic, few times a month).
DD1 is not a problem as she is tall/heavy enough for a normal booster but DD2 is only average 3 year old size and 12.7kg, so too small for a high back booster that will last, and it's a waste of money we haven't got to buy one that will only last a few months.

Any ideas?

Britax do one. last from approx 9 months to 11 years think it evolva123. Five point harness then full back booster with three point belt. Both my boys got them. Have great support and side impact.
hope this helps

thanks, i mucked up my op, I meant she is too msall for a NON high back booster but DH's new car doesn't have the room to keep a highback in all the time. She has one in my car already.
I might just ban her from his car until she grows

How about the Kiddy Infinity Pro? I think it's smaller than most high back boosters and will do until she's 15kg. It's a group 1 seat but has an impact cushion rather than a harness so hopefully she won't see it as too babyish.

Recaro young sport. She still needs 5point harness or a impact cushion

Loopy, by highback booster I mean the ones that go from 9 months to 12 years [http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_780553_langId_-1_categoryId_165763 like this one]] so she's fine.
THanks for all advice!

balls