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Design question for a tree house for a summer camp

The summer camp I work with is a small non-profit camp, (VERY non-profit :-), and we'd like to build a tree house.  We had one up until around 20 years ago when the last one rotted badly enough and hand to be taken down.

The problem I have with a lot of designs I see on the internet is I get the feeling they are intended for a family with 2 or 3 kids, (who are probably 5-10 years old), and that's about it.  My camp has 125 boys ages 7-17, and whatever I build has be sturdy enough to support a whole bunch of hyper boys, some who might be large, filling it up.  

I can see some people saying "well, just limit it to 4 or 5 boys at a time".  Our camp doesn't work that way.  We have "free time" where all the boys can roam around the camp and do whatever they want with very few adults anywhere around.  The location where we build this tree at will be relatively rare for an adult to wander by and say "there's too many boys up there".

So I want to build this tree house VERY sturdy.  We can't afford a professional, (I'm going to be funding most of this out of my own pocket), so getting an engineer is probably out of the question.  What I do want to do is over engineer the thing myself, even if costs me more money.

I'm thinking about building something like this:  
(including the fireman's pole!)

But in looking at the design I  worry that it's not sturdy enough if a whole bunch of boys pack into it.  I want to add things like two of these tri-beams using TAB's:   They key though is that I'd make them stick out much wider than the width of the tree.

I'd put one tri-beam on either side of the tree, and then put two 20' long 2x10's on top of them right next to the tree.  I'd sink 1 large lag bolt through each of the 2x10's and bolt them into either side of the tree.  Then I'd have 2 more 2x10's further out on the tri-beams, and fasten them on using standard construction fasteners.  Then I would put all of the regular 2x6 'risers'(?) on top of those four 2x10's, and then put the flooring on top of those.

My thought was that the 4 TAB's on either side of the tree, plus the 2 large lag bolts on the other 2 sides of the tree, should give me enough load bearing capacity to handle MANY hyper boys.  Plus having the 2 extra 2x10's further out on the tri-beam would give me more side to side structural support in case they all more to one side for some reason.

The tree we will be using is a healthy looking oak tree.  No idea how old it is, but we don't see any obvious problems with it.  We heard you can do a core sample to see if it's hollow, but we have no idea how to do that.  The thing is since I will be paying for this I want it to last a LONG time.  That's the reason I'm over engineering it, and hopefully on a tree that will last several more decades.  Other than the TAB's and the lag bolts nothing else will be nailed/bolted onto the tree.

I admit that I don't know anything about what I'm doing, (other than watching lots and lots and lots of youtube videos).  Any suggestions would be very welcome, (other than to hire a real engineer).  :-)



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