Seriously.... do these people love your child? Are they good to your child? Do you think they genuinely care about your partner, you, and your child... or are they evil Sara Lee lobbyists, intent on shoveling cake into anyone who will open their mouth, including your child?
Look, you have a right to raise your child any way you see fit -- I respect that... and I know it is not easy in a world of *mainstreamers* to stand up for and reach your ideals.... but I would let it go.
What is it you want to teach your babe? Honesty? Kindness? Respect? The incredible quality of standing up for her boundaries while also leaving another person feeling valued? I am sure those things are WAY more important to you than dying on hill over a first birthday.
You have gotten great suggestions. Chances are, the grandparents just want the memory/experience/tradition of seeing a babe get messy on their first birthday --- so explain to them kindly and respectfully that you won't be having a cake but will be having *whatever* -- there are tons of recipes for raw cakes, vegan cakes, fruit sweetened, wheat free, gluten free, whatever free you want and chances are your kid won't even eat but a bite or two of it. We had a vegan cake I made for dd for her first and I think she had like, 2 bites... got a bit messy, everyone was happy, tra la la...
The frilly dress is a matter of style. Is there no style in the WORLD they would like other than a baby prom dress? I am guessing they just want some cute pics of their granddaughter... that is not a crime. Yes, this is your child, you can do what you want with her but others like to share in a tiny bit of that joy too and I am sure there is an outfit you would approve of and a photo place (or personal photos or whatever) that would suffice.
The toys -- be honest... simply... be honest. That seems to be lost on this board sometimes "tell her this or that or the other!" -- Just be honest. "We love that you want to give so freely to our daughter, and we appreciate how much you want to see her happy.... would you be willing to buy her wooden, natural toys like *example*, *example*... " then add your reasons. If they don't and continue to buy plastic, well, you've told them and I wouldn't feel a bit of guilt about anything (donating etc). No one is a mind reader ya know?
I just think honesty and sensitivity go a long way with people who are basically good hearted as most grandparents and such are. They have good intentions, they want to express love, they want to express their love in the only way they know how and should at least (imo) been given the benefit of a head's up....
Now if you've given them the head's up about your life and they just give you a big eff you and do what they want anyway, then well, not so good --- but at the end of the day, smile, nod, and do what you want anyway!
My family is surprisingly AWESOME in that area ... on both sides. For a long time though, I felt like you did -- all resentful and up in arms and posting on mdc about my plastic loving mainstream family

but the moment I was honest with them, explained kindly and respectfully, told them my reasons -- expressed to them that I know they love us and I appreciate so much their expression of that in things they choose...however (then explainted things etc)... they were so receptive and but for a *slip up* now and then, they are so great.