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......Tommy Tuppence 2000-2012 and a half share of Faith.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.