Well if it is a muslin and not a final product, I don’t think it will matter too much. After all the point of the muslin is to get the sizing/fit right anyway.
You know her growth pattern best. Is she due for a spurt, is she a little weed in the sunshine, is she a slow and steady grower, did she just have a spurt and due to have a “resting” period?
I’d consider the 18-24 for the muslin. But do the BODICE only as the skirt isn’t really a big deal. Just the rectangle which is easily altered if too large. Just hem it more. No one will think that if the rectangle is too wide it has too many gathers, especially at that minimal size difference.
But do a muslin bodice. That will tell you if it is too baggy right away, and then you can do a muslin of the bodice for the 12-18 if you think the first one is too big.