Hiya Carol!
Welcome to planDisney! I love magical friends who bring home some pixie dust for us. It's like having your own Fairy Godmother!
I don't think there's a more classic Disneyland Park souvenir than the Mickey Ear Hats! I just went on a little hunt for all our ears across my home to get a better idea of sizing for you. The hats usually come in "Infant" "Youth" and "Adult" sizes with the diameter getting slightly larger with each size. The bowl of the hat also gets a little deeper as the sizes get larger. I think a youth size for your younger grandchildren will be excellent! My son is about the same age as your oldest grandchild, and he can wear adult-sized ears now, so that would be my recommendation for the 11-year-old. Since the hats typically have a chin elastic to help them stay on it's not the worst thing to have a little wiggle room!
While there are many places across the parks to purchase ears, I always recommend
The Mad Hatter in Fantasyland. The shop is so whimsical! There is also the option to add embroidery to your hats, the classic cursive style is my personal favorite. Of course, the Mad Hatter himself is a little, well, mad...so he has expanded his hat empire to include a
store on Main Street, U.S.A. as well. The most convenient option for your friend might be to purchase the hats there.
Make sure you tell your friend she's welcome to
check her purchases at participating merchandise locations so she doesn't have to carry them around while she enjoys the parks! It's nice to not have to worry about bags or accidentally leaving something behind.
I'm sure your grandchildren will look darling with their ears on, Carol!
Keep moving forward!
Amanda