It is a good idea to buy ponchos for the little ones before you go. The adult ones are a toss up. The ones at Disney have Mickey on them, which to some is a silly reason to spend extra money, but might make you want to wait. Here's a good tip-if you are using a stroller at all, you want to cover it up before you go into rides and shows in case it rains while you are gone. We bought a stroller cover for our double, but discovered that an adult poncho totally covered a single, umbrella style stroller. You may want to bring along an extra adult sized one for that reason.
That's great about the extended nursing! From what I've read about nursing rates in England, you are a hero

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