Flower girl problems

My future MIL is obsessed with making my flower girl (her granddaughter) look like a miniature bride. It makes me slightly uncomfortable.

Don’t get me wrong, the FG dress she has picked out is beautiful, I just think it’s too similar. It has a very long train and she wants her to wear super high heels that I think are inappropriate for a 6 yr old. The flower girl dress is also white, while my gown is ivory.

I haven’t said anything yet because my fiancé’s mom is hard to reason with and things might go more smoothly if she has some aspect of the wedding that she’s in charge of.

Should I ask her to change the flower girl dress, or would it be best to leave this alone? Is there a tactful way to go about it?