Forgot to say - they do come at quite a cost compared other solutions - but we went with them specifically because we could minimize the number of APs actually needed to cover the area.. ~20 arrays compared to ~100+ APs..
I'll second the Xirrus arrays as being a good option. We use them in a school environment - so very different to what you require - but they do have several features which may be of use to you... The arrays all come with multiple radios (4, 8, 16 etc) and some of these can be configured to 'back haul' your data with a directional antenna while the other radios in the array provide a multi-directional network for your clients..
The outdoor enclosures also look really good - temp controlled and a rugged design that should be able to take a beating from critters/kids etc
Obviously trees / etc will cause nightmares - but with careful placing we've been able to serve multi-story solid concrete buildings with a single array (providing plenty of throughput for ~100 clients all doing network intensive stuff at ones) - so your mileage may vary... But they (at least in Australia they do) offer a no-commitment site survey which was better than I was offered from the other big names resellers when we were researching last year..