S.H.
Wow, what a huge thing to invite all 30 kids... think down the road.. will you ALWAYS invite his WHOLE entire class to all his birthday parties? At this age, it is never expected that ALL kids HAVE to be invited.
At my daughter's school, the Teachers always explain that IF any child has a b-day party, the whole class does NOT have to be invited (it's a financial hardship for lots of families or an obligation type burden), and invitations can be given to the Teacher, discreetly, and the Teacher will pass them out... privately. This is how it is handled at my daughter's school.
As far as asking for Donations... some think it's tacky, some don't. So it's up to you. Also, some may not "like" the charities "you" chose... and some may not like just giving "money" to go toward a charity... etc. ALSO, I went to a party like that once for my daughter's friend... and EVERYONE STILL brought birthday gifts with them for the girl, IN ADDITION TO SOMETHING FOR THE DONATION request. SO, therefore each family brought TWO "gifts" for 2 purposes. BUT... the select few that did listen to the party's directions/request for only a "donation" then felt awkward that they did not "also" bring a gift for the birthday-child. So...there is awkwardness for these people.. and they may feel "excluded" socially. So there is that factor too. AND, some people go all out and bring HUGE donation items/money... and then "this" makes other people feel even more awkward that they could not bring something equally "lots."
Also, at 30 invites... some people may see the "birthday" party as a mini "fund drive" kind of thing, versus a birthday party for your son.
So, think it out, first. People being people, may still often bring gifts for both.
All the best...
Susan