I have taken care of other people's cavaliers many times and I am sorry to say they are very fickle. They usually settle very quickly and transfer all their devotion to the new treat giver. You will probably be a lot more affected by the parting than they will.
I agree that the unfenced garden would worry me and I would prefer a secure kennel environment, where they could be kept together.
I would make sure the new address is very visible. In the past I have ordered brightly coloured plastic dog discs on eBay and received them very quickly, but you may not have time?
I have found that the barrel-like tags are often mistaken for ornamental pendants by the non-dog-owning people. They do not realise there is an address inside.
At a pinch, a thin label laid along the collar & completely secured by yards of sellotape will last and remain legible for weeks.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi