Senora, I do sympathise. I have been looking into booking a car today, picking up and dropping off at different locations in the uk. With the airports and times I was thinking about, it couldnt be done. The flight arrives too late at bournemouth airport for anyone to hire me a car. I could do my trip the other way around but flying into Manchester was way too expensive and the flight times back from Bournemouth meant cutting the holiday short by 3 days. I think im not going to bother going to England and spend my holiday here. [frustrated icon needed]
perhaps I could come and house sit, hahaha . To be honest this will be my first visit back to England and I don't know if I can face the tour of duty. It will mean having to cross the whole country really. My immediate family are here. Anyway enough hijacking for one night and back to your dilemma, could you fly in and stay in hotel over night and pick up rental car in morning or get a relative to hire car for you and pick you up with car seats? If I do go to England, that was the only way round it if I wanted to fly into bournemouth at 12 o'clock at night.
carwithaview - "'Car with a view' is an online car hire booking agent for Hertz, Budget, Alamo, Dollar, Thrifty, Europcar and Avis. For families hiring a car extras such as child seats, roof racks and other accessories are available at many car hire branches and are charged locally in UK."
I know that it doean't help, but I have generally found that you have to book the car and then arrange the car seats once you have the booing reference. And even if you do manage it all online I would still phone the pick-up location and confirm.
Having travelled on my own with 2 littlies though I can confirm that actually booking the hire car is the worst bit, and I've usually found that staff at the terminals are very sympathetic (and help getting luggage/toddlers to the car etc). They will insist that you check the car seat installation though (which i would anyway!).