I would look at this way:
You can't control them, you can only control yourself and how you react.
And of course, you get to make the choices for your baby and your home.
So, do they have the choice to buy the newest off-gassing plastic gadget for your baby? Unfortunately, yes.
Really, you cannot stop them and will only stress yourself out and take time out of your life.
But at the other side of that is- You in *no way* have to accept it or keep it. You have the total right not to allow it in your home, and not to allow your baby to have it. Does that make sense? But trying to control their actions is going to end with you banging your head against the wall a lot, IMHO.
There is a book that I always hesitate to recommend because here it is "Christian," but it explains it really well. It is called "Boundaries" by Cloud & Townsend. It kind of lays out exactly what you can control (your choices), what you can't control (other people's choices), and lays out what works.
Really, your parents have a choice. They can buy your baby loud plastic toys all they want, if they like to see you refuse them or drop them at GoodWill on the way home. OR they can make the choice to buy your baby simple wooden toys and see you gratefully give them to your baby. If you can just emotionally disengage/detatch and let it be that *simple.*
It's really all in how *you* say it, how calm and matter-of-fact you are, etc. You don't even have to remind them anymore. If they come over and ask "Wheer is that light-up flashy obnoxious thingy that was made in China with lead-based paint?" You can just look at them calmly and say, "Oh, I thought I told you we don't allow those for our Baby. So, would you like to see the latest pictures of her or hold her while I get a snack?" and smile. Change the subject just like that.
The thing that really pushed me over the edge on this and caused me to learn about boundaries and how they work, was my mom. She would take DS and his carseat (which we would install for her in her car) and then not put him in it, and let him ride on her lap. So we said she couldn't take him in a car anymore, after going around on this and her lying and getting caught. So she went and got a carseat and put it in a prominent place in her home, basicly saying that she knew we would 'give in' now that she had a carseat herself. (It was totally irrevelant, because the point in the first place was *not* that she she didn't have access to a carseat, it was that she would take it and then lie and refuse to use it.)
So anyway, it drove me nuts to see it sitting there. Then one day it occurred to me, you know what? She could buy 100 carseats, and have my DS's name engraved on them, and tell everyone in the world what she had done, and even buy stock in carseats. But because she wouldn't use it and we knew that (her choice), she still wasn't taking our DS (our choice). That was our boundary. We calmly enforced it. We didn't need to get upset because the control was ours. We just had to be consistent and firm, and not apologize or over-explain or try to win them over... We just had to smile nicely and say, "No, we have a carseat rule, so he can't ride with anyone who won't follow it. So are you doing this weekend? I heard you were invited to......."
Once we actually had to call the police on her to keep the boundary, because she has mental issues, but for most people it doesn't escalate that far!
And really, be sure and pick your battles. It is pretty easy to drop off some plastic junk at GoodWill for otherwise awesome grandparents, as opposed to them feeding your baby Pepsi when you turn your back, or something, yk? Also, really try to see things from their point of view. I am sure I will be nervous when my baby is having a baby someday, no matter where she chooses to give birth! And honestly, I am sure I will want to see my grandchild before 30 days (regardless of whether I get to or not). Not saying you have to do anything you don't want to do, but if they are otherwise loving and good family, I would really try and overlook some things and try and imagine how they must feel.