I love all the ideas that have been shared in this thread, including the OP's original invitation wording which sounded extremely kind and welcoming to me.
I have mentioned this idea to my 7 year old over the past couple years (doing a donation instead of gifts at her birthday), but will not do it until she comes to a point where it is her choice to do this. Personally, I think the message is lost if I strong arm her into giving to others, and she is an extremely giving person with her allowance so not a completely lost cause. I will ask her about the theme parties that were mentioned earlier. Maybe they will be more appealing to her!
Also, I would love to receive an invitation that directs me toward a gift, especially if it will be affordable! In my opinion, it is extremely difficult to buy for a child who you do not know (from my daughter's class for instance), and then not feel pressure to spend over your budget.