I think it all depends on the way you would have to display the ads- at the moment the forum looks a very trustworthy source of information- it has a good, open image.
I would be concerned that an ad-presence on the site would perhaps make people wary that the content they are reading may not be entirely impartial (especially as google ads tailor their ads to the subject matter of the forum- you probably won't have control over who can advertise on the site.)
In principle I think its a good idea/good way of raising money for charity- just depends how much control you have over it...
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