4.5 hours should be fine - it should take about an hour transit time once you reach the Tube station in Heathrow, which depending on the terminal could be a short elevator ride down or a long walk (terminals 1, 2 3 use a unified Tube station that is a long poke IMO)

The deposit is not different if you buy online - it is now £5 per card plus I think you would need to pay for shipping (if you can even buy a regular oyster online from overseas I am not sure). There really is no benefit to doing it before.

I just went to the website and I think you are looking at the visitor oyster? Looks like £3 deposit but I cant change the delivery address to anything other than UK...so I suppose if you can get the delivery address correct it may save you a few ££...

Thanks PalenQ!

yes, jamikins, I was looking at the visitor oyster. I didn't get to the part to see how much the shipping would be so if it ends up eating the savings, I may as well just get them on arrival.

Not to mention, I am still somewhat confused on how to get the kids theirs. My daughter is 17 and my son 13. however, it seems like she will fall under an adult oyster card and that it may be better just to get my son 3 one day travelcards?

hmmm, will most likely just get them all on arrival so that someone will be able to advise my on what is best to get for my situation.

Correct me if I'm wrong but I'm pretty sure you can't get your deposit back on the visitor Oyster card, you can on the normal card. There was also something about not being able to load travel cards onto a visitor Oyster.

you can buy oyster cards on arrival at heathrow at no extra cost which may be easier