What should my 7 and 9 year olds wear to a christning?

My nephew and his wife are having there baby boy christened at the beginning of July. I have 2 daughters aged 7 and 9 years. Any suggestions to what they could wear. I would love get my girls white dresses and white shoes. Is this over the top??? .. My girls do like pretty girly dresses. but not too sure on the white... after all its not them thats being christened.. also .. should I buy them matching outfits... or get them something completely different each.. or contrasting... ideas please