I made the 6year old size for a nearly six year old, and they fit just fine. Wool is tricky... I think it may be some Wendy chunky as I have the paper wrappers for this and the colours seem right. I'm usually so much better than this at keeping a record of wool used! So, Wendy Serenity chunky it is in colours ochre and Tarmac.
These were fun to make until the sewing together bit, that was a little fiddly. They would probably have looked neater finished in the same wool but I thought I was going to like the contrast, and also by this time (these were actually the last slippers to be made using this wool) I had used up all the ochre wool.
Happy birthday, Pippa!
And I do still want to make this pattern again, this time in all their beastly glory...