Sunny Day shorts

I've made loads of these for my grandchildren, I love the pattern and have recommended it no end of times. Howeveer, I'm coming unstuck on the oldest one, now 7, she has always been very tall for her age, now over 4'6", and is also now quite shapely. The largest size on the pattern is 12. I can easily add to the length, as I think they'll be a bit short for her. But I'm not sure whether even the age 12 will be big enopugh for her (assuming I'm using woven fabric). There's no finished hip measurement given, only a finished waist, which is shown as 31 1/2" extended (i.e. with the elastic stretched out).Given that her hips are 30", that seems a bit tight to me. There isn't any shaping on the hips at all, so I don't think there'll be enough spare for her to sit down in them. Any thoughts? Or are they just not suitable for girls once thewy have hips? Is there a better pattern?