Thanks for your replies.
Karma, you've summed up exactly how I felt / still feel. That maybe it was more about what DP wants, than what DD might want. After all, she's not going to care where she is really - but it might bother her, that the person who is normally there 24/7 isn't, for 3 straight days and nights - especially when in a new place / new bed etc. I thought it would be enough of a change for her already for me to be in work during the day, without her not seeing me at bedtime and mornings too.
Knitted, although I admit I will (of course) miss her, I don't think it is all about me, me, me. Do you not think it'll bother her at all being away from me and in a new place? (Maybe not - I don't know)
I didn't say anywhere that there were things I could do that DP couldn't, but as Karma says, she is still at an age when she often wants Mummy rather than Daddy - eg when she's tired, hungry, upset. That's not to say I want to encourage this, I don't - I just wonder if being away from me for 3 days all at once may be too much.
Diddl, he hasn't booked anything yet, just thinks that it would be a good opportunity to go when I will be busy anyway - maybe he's right. I don't have a fantastic relationship with them, but would go if I could and it would be fine.
And Mr Spoc, I've got no idea about your wife's expectations when you look after the kids - but of course I wouldn't expect DP to stay home all day with her - that'd be weird! I'd fully expect them to go out and about and do fun stuff - just think that maybe heading off abroad without me, is a bit at the other end of the spectrum.