I too agree she's too young to be a plane ride away, especially if she's never been away from you before.

When they visit, do they take her out for the afternoon/day? It's something you have to work up to, iykwim? Start with you ducking out for an hour for groceries, then build up to a half day, a day, or an overnight stay with you nearby. Take it as slow as you need to and don't feel pressured. Ever. It's your right as parents to parent as you want.

I'd be getting DH to tell them too, and to say its a joint decision. Also get him to say that their constant asking, while showing how much they care, is making you both feel uncomfortable and pressured. Let them know it's not them, that DD isn't ready to go away with anyone just yet.