I'd probably just include a simple, to-the-point request on the invitation, like:
"In lieu of gifts, Jane requests that donations of non-perishable foods be brought to the party."
Depending on how much you want to say, you could follow that with: "Please help her realize her birthday wish of stocking the Food Pantry's shelves with some much-needed items."
Or add to the beginning: "Jane has more toys than she knows what to do with, so this year, in lieu of gifts, she requests..."
Good luck!