"A Rhyming Baptismal Prayer" by M.S. Lowndes and "Baptism" by Claude McKay are two good examples of prayers for Baptism. "Baptism" by George Herbert is another.
While these poems are intended for baptisms at all ages, there are also several good poems for baby christenings. Some of them are "The Greatest Gift of All," "Christening Poem: Sweet and Low," and "Baby Christening Poem."
Bob Dylan also has song lyrics that are used for christenings. The song is called "Christening Prayer," and the first two lines read, "Welcome, precious little child, so divine from God above; Christened today in Jesus' name, held in His arms of love."