Pleas don't wear tight/revealing 'nightclub' type clothes - it's just my bugbear, I * appreciate* that people who do this are wearing their 'best' outfits, I get that, but it's just so innappropraite for a Church/Sunday afternoon, and I see it a lot at Christenings lately..
I would say ask if it's part of the usual sunday service. We went to one a while ago and got all dressed up for it but the christening was part of the usual sunday service and all the regular church goers were there in jeans / t-shirts etc. We looked like right twats all dressed up to the nines!
I'd go with something smart / casual - more on the casual side!