I just don't understand what kind of set-up/PROVISIONS the bride and groom are going to be making for the guests and kids, apart from giant Jenga? Is all food and drink provided? Will you have to bring your own tent, or are you camping, disaster-refugee style under one giant circus tent in rows of camp beds?!
I've taken my toddler to a festival. We mostly had a brilliant time, but we had to cart a hell of a lot of stuff, there was a kids' field full of stuff to entertain her, a funfair, helter skelter etc etc. It was in August, but an unseasonably cold weekend, which added to the volume of stuff we had to bring to ensure we didn't all freeze to death. We brought most of our own food, but there were plentiful food stalls, bars, bins, composting toilets, etc etc ETC. And even with all that stuff on hand, it was hard work when our kid woke up at 6am and one of us had to throw clothes on and get her out of the family camping area and into the main arena before she woke everyone else up.
I think if I were you I'd just say that your daughter won't sleep well in a tent, so you'll all be miserable in that case, then go for the hotel and turn up washed, fed and nicely refreshed for the festival-style 'fun' during the days.