Car Seat Questions

Yes, they do rent them, at about £10 per day.

https://www.avis.co.uk/drive-avis/car-hire-extras/child-safety-in-cars

There was a TESCO (similar to WalMart) in Slough the last time I went to Heathrow/Windsor, so they could be purchased there, but renting would be easiest. I know how sleep-deprived one can get on those flights, and I can't imagine doing the trip to a place as gigantic as Heathrow with 2+ kids, so the low-stress travel rules probably come into play. Rent them since you're only going for 4 days, and have that many fewer things to think about. I would not reccommend driving to a store to go buy car seats when jet lagged in the giant city they call Heathrow, while also driving on the other side of the road, other side of the car, and with 2+ kids after that long of a flight. If you're checking bags anyway, and need to save the money, maybe you can check your booster seats too, but I wouldn't reccommend going to deal with checked baggage for the booster seats alone. It's a huge place, the incoming international passport-checking line alone is enormous most every time, adding baggage to it when not necessary is no fun. Less is more. :)

Are you going to take them to Legoland in Windsor? A high school friend of mine married an Englishwoman there and took their 2 kids recently, they're adding a TON of stuff, they had a great time.

Happy travels !
-Alison

Just to add if your 8year old is 135cm or more tall they are not required by law to use a booster seat. Personally I've always checked in the simple boosters (no back) as I've always had a suitcase to check in anyway and have never found it difficult. I've had to drop them off at a different location but they always arrive in baggage reclaim at the same time. If your youngest is over 25kg these no back seats are allowed if you bring it with you (although you can't buy it in a shop as the rule has now changed to 35kg)

Bring your own. You never know the condition of the ones at rental places ( learned this from experience) and the last thing you want to do is run around looking for a store that sells booster seats

I, too, suggest bringing your own, as you never know what the rented seats have already been through; i.e., an accident making them less safe. Exiting arrivals you have to walk right by the luggage carousels anyway so there is no confusion added to it. Traveling with kids that age, I'd check as much as I could anyway to lighten your load.

PS, Heathrow (T5) is my European airport of choice (over CDG and Schipol, at least). I find it so much more pleasant and easy. So don't necessarily get nervous about it, as we each experience them differently and you might as well determine where you fit, if you don't already know.

we swear by these for travel: https://www.bubblebum.co/us/

they take up little space and our children say they are very comfortable, too.