I think it would be an honor to make the christening gowns for your twin grandbabies. I know the time frame is short, but if you think about what this will mean to their parents, and to them as time goes by, and to their children too...
I offered to make a christening gown for my granddaughter who was born nearly a year ago. My daughter and son-in-law very politely said no. The reason was that my son-in-law had a christening gown in his family that was made for his great-grandfather, then passed down to his grandfather, then to his father, then to him and now to his daughter. The gown was beautiful--all hand-made lace, and hand-sewn. An heirloom that is still being used, and I believe will be for my granddaughter's children too.
Just think, you could be the one making the christening gowns for your great-great grandchildren. Oh--I wish I could have had that honor...