Have to say I'm not a fan of pink myself. I prefer blue. :K But if it must be a pink sort of colour, I would rather see something like what Mel posted rather than a pale pink.
I think Baynes' illustrations do a very good job. I don't mind changing it up a bit, but I'd definitely want it to have the same Victorian feel.
I also wouldn't mind if Polly's dress subtly references Alice's Adventures in Wonderland. After all, Digory and Polly do go find their way into a rabbit-hole of their own.
Maybe something like one of those? Possibly the dress on the right? Minus the hats, obviously. The stockings and boots are pretty close to what Baynes drew, I believe. Not super keen on that style of collar, but that can always be tweaked a bit, for sure.
Maybe a collar more like that, perhaps less frilly, but that shape with the knot? Baynes' illustrations definitely included the knot, as her Polly was in the sailor-style dress.
I'd like the hairstyle very close to the original. Maybe something like that? Can't tell if the Baynes version is a headband or a ribbon, but it definitely looks like it serves the same purpose as a headband. That girl looks like Polly to me, though, for real!
Also, I just have to say, I know we have fun discussing costumes, but I live my life in shorts and t-shirts and I can't imagine having to wear this stuff, lol.