I'll answer my own question in the absence of the OP.
Someone who has a daughter aged 52 is probably in their seventies, at least. Transferring assets into children's names is a popular, and shit, way to "avoid inheritance tax". Unfortunately, if you transfer a large asset out of your potential estate to others while retaining some sort of hold on it (such as an agreement to have it back on demand) then it's a "gift with reservations", and may fall back into your estate upon your death.
The scheme, I suspect, was that the agreement would be conveniently forgotten upon the OP's death, so that the holiday home would pass to the children outside the estate and therefore outside IHT (which, thirteen years ago, was more onerous than it now is).
But the reason people are advised against these scheme is that, aside from potentially being ineffective, they are a nightmare if circumstances change. MSE's forums are full of, for example, people who transfer their house to their child with an agreement the donor can live there rent free (itself a "gift with reservation", of course). The child then gets divorced and the donor's house is then part of the marital assets.
This scheme was doomed. The holiday home is part of the daughter's estate, so if one of the children pre-deceased the parents, the agreement would lapse and could not possibly bind the deceased's executors or heirs. It isn't an effective IHT shield. And, worst, if the recipients refuse to abide by the agreement, there is a very, very messy legal action involved in fixing it.
The OP needs professional legal advice, pronto.
Others thinking of engaging in schemes like this need professional legal advice and their bumps feeling, pronto.