I haven't seen a Mary Poppins doll...but there is this etsy shop
that sells a lot of doll patterns that you may be able to adjust to Mary...she does offer this one for free
and you may be able to adjust it some way (under the doll picture is a link to a dress for her).
That is going to be a fun themed party, I hope you find a doll that fits!
Oh I just looked at ebay
, it's not knit but here is a doll pattern
There is also knit patterns out there for her scarf