Just a brief bit of advice...
It seems that Amazon UK have sourced the "normal" width Birkenstock children's sandals.
These are phenomenally wide! We had to return the (size 30) sandals immediately, there was a clear finger's width between the side of my daughter's foot and the straps. She usually measures up as an F fitting in Clarks etc, so not exactly unusual.
Having tried some "narrow" fit sandals on the High Street the width was much more typical. However a couple of shoe fitters said that the sculpted sole of the Birkenstocks is not well suited to the growing feet of children, because in no time they are misaligned with the toe bars.
It's a shame because these are great looking shoes which her mum finds very comfortable, but they are not for my young 'un!