What would I do about car seats?

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I want to visit my grandparents with my 9 month old and my 3 year old. They live at the other end of the country and I don't want to do a 10 hour car journey with them by myself.

I could hop on a train and think this could be rather fun. At the other end though they live 1 hour away from the station and don't drive. Can I hire car seats when I hire a car? What do other people do?

You can hire car seats with cars in France from the big car hire companies so I imagine you could do the same in the UK? I would check their websites out.

Yes hire seats

Yes you can hire them from car hire companies. But can't guarantee quality.

For the cost of train tickets and car hire, I would be tempted to drive 5 hours, stay in hotel, then drive 5 hrs the next day. That way all luggage can go in car, and you can keep regular car seats etc.
They will sleep a section of journey in car when bored! Add some story tapes and your good. If hotel has pool half way even better as can take them for a long swim in morning on second day so they are tired in car

Yes you can hire car seats we've done this for years as we fly to my parents (opposite ends of the country + not enough leave to spend 2 full days travelling) and they have always been fine but you might if you do this when they are bigger (yours are too young right now) want to consider something like the trunki boostapak which is slightly more cost effective than always hiring seats.
When DS was a baby I put him in his car seat on the buggy, on the train, so I had both when we got to the other end & I had something to carry all the kid paraphernalia & luggage on whilst negotiating the station.