The reason behind people naturally pronouncing "Ceri" as "sair-ee" is that in English, a "C" before an "E" is traditionally pronounced as an "S". I find it very hard to believe that so many people believe that others should be well brought-up in Welsh as to articulate it as "care-ee"/"KE-re", or the such.All in all, unless it is pronounced "sair-ee", I really am not a large fan of this diminutive... nor am I a fan of "Ceridwen", by any means.
I agree it can only be pronounced 'Kerry'. The Welsh 'C' is only ever pronounced 'K'. If you want your child to be named 'Serry', name them Serry. Not difficult.
Being fluent in Welsh and having several Ceri's at the school which I attend, I would like to point out that Ceri can NOT be pronounced Serry - only like Kerry.