After it's written, and you're polishing, think about vowel sounds. There should be very few words or lines which end on hard consonants (K, T) because you want the sounds and the lines to flow in a stream.
Say your lines aloud in a singsong (never mind a melody) to make sure they can be sung, and you haven't picked a word which stops on a voiceless syllable.
Example to make that coherent:
Aaaaare you sleeeeepingggggg,
Morning bells are rinnnngingggg,
Dingggg dangggg donggg.
Note that the emphases are on vowels and the voiced N and NG sounds.
In "Rockabye baby," the second line does end in "rock," with a hard K sound, but when it's sung, you always emphasize the O of "roooooooock." (not to mention I think it's a horrible thing to sing to a child anyway, but YMMV.)