Mine went to stay with Auntie Tracy at Rabbit and Guinea Pig Welfare . But I did go and see where they'd be staying before I booked them in because I'd never met Tracy until that point (I'm very fussy ). As soon as I saw how spotlessly clean and well looked after all her 'inmates' were, I could tell they'd be in safe hands. She also came with a recommendation from our vet (we see the same vet).
I think it's essential to check out boarding facilities - especially if they've not been recommended by another bunny savvy individual. You need to see where they'll be staying and to know that the person caring for them totally understands about rabbit health and behaviour. And I also think it's important that the bunny boarder will take them to a decent vet if they're ill. Don't be worried about asking questions or asking to see their facilities - if the boarder is loathe to answer, can't answer or refuses to show you around then run away as fast as you can and take your precious babies with you.