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).
do you let nayone else take them out for the day? I don't let dd with anyone apart for my mum so have no problems saying no- point blank- i don't really care if i offend anyone. a friend of mine asked if she could take dd for a walk and i said - not without me. Either say no or just let dd1 go. I honestly think that 3 kids is an awful lot for someone else to handle.