My wife and kids are due to fly from Kiev via Frankfurt to Heathrow with Lufthansa. I meet them at Heathrow and we all fly British Airways to Miami.
They all have US visas for their trip to Miami. My question is based around the fact that they won't be able to check their bags from Kiev to Miami and will have to collect at Heathrow. Will they be able to do this without any sort of transit visa?
The flight is in a week and I am just became aware that this could be a problem.
Assuming your family are Ukrainian citizens; the departing flight leaves the same day or the next day; you're flying to/from the USA with valid visas then you should qualify for transit without visa.
See the UK Government visa web site for full details.
Note that your family could be refused entry by the border guards if they consider that there is a problem. The way to avoid that is to apply for visas before they travel, but I guess you don't have time for that now.
Good luck!
At the UK Border, they should present their passport open at the page with the US visa, the filled-out landing card (blue form usually distributed on the plane) and their connecting boarding pass (or, if not possible, e-ticket confirmation). Then explain that they have to collect luggage.
They will be admitted until 23:59 the next day.