I actually think they are safer where they are, Wallcat. Cats in long term cattery/rescue situations do get used to it quite quickly, and although there's still months to go before your son can fetch them home again (I'm wondering if it's flood damage?) they are at least safe and well.
The other problem you may have of course (aside from them scooting off and never being seen again, that is) is the potential affection the temporary guardian may attach to them - you could end up with broken hearts too.
I have 8/9 cats at present and know that it takes very little time for one to become bonded with these little souls.
I wish you luck whatever happens.