Here’s the thing: your grandchildren WILL be raised interfaith.
I’d strive to make Judaism the safe space. The thing she can come back to with a broken heart or complicated friendship. An oasis. Because even when kids seem to be pushing away, they also appreciate comfort, the familiar.
Times of loss or doubt are exactly when a religious community is most important. Not because anyone else can reassure you about the afterlife, but because loss and doubt are truly isolating, and it’s important to know you aren’t alone right now.
She’s far likelier to listen to you, and respect your questions and suggestions, if she feels like you’re listening and respecting. So do explore alongside her.
Your kids will probably handle camp the way they handle other experiences. If one of them is often anxious or shy in new environments, that will probably happen this time too.
I need to be very honest here: I’d be one of THOSE people. So I’m struggling with how best to answer your question.
Maurice Sendak’s books — classics now — show children wrestling with difficult emotions, and they challenge adults to embrace high adventure.