Taking your classmate's rudeness out of the equation for a moment, what criteria will be used in assessing the success of your work, and your grade for same? Is there any possible validity to the points that focussing your proposed project more or exclusively on children would increase the success of your project being funded (understand it's a hypothetical, but presume it will be evaluated on real world factors)?
It may also be that your instructor is not getting involved in the dynamic between you and your classmate because exactly this kind of problematic communication, and differing agendas is part of many grant proposal processes, both in drafting, shaping programs and obtaining funding.
Your classmate does sound unpleasant, but perhaps it would help to consider your interactions and communications with her as a difficult but valuable opportunity to practice how to deal with such people in a business setting (we all do to varying degrees) and approach it as though you need to either get her to "buy in" to your proposal, or successfully prevent her from derailing it. In other words, don't think of her as a classmate, think of her as though you are pursuing a real project, and she is either a board member of the museum, or a patron with ability to provide funding approval.