A bit of background, because I'm genuinely not sure if I'm the one being unreasonable here.
H and I haven't had a real holiday since the kids arrived (weekends at parents excepting). In the Spring we've planned to stay for a week at this hotel which we chose because it's quite family friendly (so no worrying about food arrangements or kids entertainment), in particular because it does a lot of outside stuff like pony trekking (colleague has stayed and seemed to enjoy it) which I think will be amazing for the kids.
SIL is due to go in for some semi-routine surgery the month we're away, so I offered (trying to be nice) to take her little one away with us. The kids get on very well together, so she would have a small break for recovery and not have to fret so much. So far so good.
BUT... the problem is that, according to DH, this evening when he confirmed with her the dates we'll be away, apparently they were discussing what sort of "outdoor stuff" he meant - he explained that we want to give the kids a chance to have some outdoor fun for once since we only have a shared communal garden - well SIL has banned her daughter from participating in anything risky - no pony trekking permitted, NO swimming lessons, no hiring bikes to go on some countryside cycling - massive problem basically. Because that was sort of the whole point of the holiday!
I'm totally unsure what to do about this now. We've booked the bloomin rooms for everyone.. but we HAVE TO respect what she's asked us. But that means either stopping our kids getting involved in the stuff we'd planned (might as well just stay at a hotel around the corner 
) or letting ours go off and her daughter watch from a distance?
I don't want to be seen as the bad person if her daughter kicks up a fuss about why she can't go on the ponies or into the pool, but at the same time we can't often afford holidays and I'm upset that we might have to massively alter our plans for this one (promises already made to our kids about what they'll be getting up to).
AIBU to think that:
SIL should have warned us when she accepted my offer, before we went ahead and booked?
Maybe I shouldhave volunteered more information about what kind of kids holiday we were planning?
Maybe H should have warned me his sister was like this (we're not that close, I was trying to help her out because I would like to be closer :))?
AIBU??? (If anyone has any idea of a nice comprimise that won't result in my kids getting upset, upsetting hers, or upsetting SIL,i would love to hear it!!)