I've been there so many times, the worry place I mean!
I would have sleepless nights thinking of how my child had been lost by well-meaning relatives.
I've worked out the best way to play this is put all the onus on yourself, ie. say it's a lovely offer, child would love it etc etc, but you just can't quite yet stop worrying about your little one wondering off, getting upset, any bloody excuse.
I've only just got comfortable with school trips to the local farm yard!! (I am Mrs Paranoia though, bear this in mind).