I have tested the 10ply, large size, which:
fits my 2 year old nicely as a dress:
fits my 4 year old also as a dress
and grows to fit my 9 year old rather splendidly as a tunic:
I have also knitted the 4ply, small size which looks like it will fit from around birth to about 12-18 months old (but my sizing may be off due to not having a bub around). This garment I love to bits, but isn't as nice as it could be. I tried this yarn to see how the pattern would take to a yarn like this. I think that this would have been okay without the diamond inserts on the skirt. I love this yarn. Fortunately I have enough for another dress.