Any birkenstock experts around..quickly..? Advice for toddlers/kids

[shhhh]

Looking to buy a pair for ds (2 yrs size 6) and dd (4 yrs and size 8)...

How comfy are they..? Do you treat them the same as "crocs" in that genuine are the way forward and copies rub etc..?

Just I know dd has had copies of birkenstocks from mothercare etc and they rubbed where the strap goes over her foot.Is this the case with bs or is the material better iykwim..

also,what about ds. BS say they don't recommend for kids under 3..why..?

AND lASTLY...Do I opt for a narrow fit for both dk's..? both measure approx f fitting...

Was going to buy replicas from Next but nothing in stock and needed for a wedding next week...

I bought some for my DS (size 24, then aged 20 months!) before I discovered this fact. Ended up selling them on ebay.

a size 24 at 20 months ! ds so wanted them today but they were like buckets on his feet.. and that was them on the 1st notch !