Looks like you’ve got a tall, skinny one! I do too. Without seeing her proportions, I’d say go with the 18-24m and add length around the knee (between the rise and the hem). You can always adjust the elastic so it fits her tiny waist, and that will give her a little growing room without being too gigantic. As I recall, we graded this style so the rise would be a little longer in the smaller sizes to accommodate diapers, so I think you won’t have any problems there. Let us know how it works out, ok?
S is almost 7 now, and I could still lengthen a size 4 to fit her. But I usually opt for a 5 or sometimes 6, lengthened to an 8, just because it’s a little easier to do, it keeps the proportions more accurate, and it gives her a little room to grow.
I hope that helps!