I don't know what the difference is, between the "art desk" they got her, and the little wood table you got her to draw at? Is it not used for the same purpose?
They have been complaining about the stuff accumulating in "their" home.
As you said.
So now you did something about it. And de-cluttered the home.
But they don't like that you got rid of the things and/or the art table they got your daughter.
Maybe it cost a lot of money or they spent a lot of time looking for one, for her? Is that the issue?
But yes, you cannot keep every single thing. It does accumulate. Whether it is their home or your home, parents do go through periods of de-cluttering and getting rid of unused toys etc. That is what you did.
You did tell them you got rid of things they and YOU/Hubby got for her and from other family members and that you would be replacing the art desk with an easel.
They are upset, still.
Maybe the art-desk meant a lot to them.
Either way, it is not easy.
When one gives a child a gift... a person then has to understand that it is now someone else's. Not theirs. That is what a gift is.
Perhaps in the future... IF they give your daughter other gifts... they can keep it where they want. At THEIR house. (when you move out and have your own home). That way, the 'clutter' will be theirs to figure out and deal with.
Or maybe, you can give them the money you made off of the art-desk, per your selling it at your Yard sale? Since they are the one's that gifted your daughter with it. They still see it as theirs....
Tell them you de-cluttered their home, per their complaining that too many things were in their home. Tell them you are sorry... but you were trying to solve this issue, by getting rid of all the stuff, accumulating in their home. As they said.