The Collaborative International Dictionary of English v.0.48:

Lallation \Lal*la"tion\, n. [L. lallare to sing lalla, or
   lullaby: cf. F. lallation.]
   An imperfect enunciation of the letter r, in which it sounds
   like l.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lallation
    n 1: defective articulation of the `l' phoneme or the phoneme
         `r' is pronounced as `l'
    2: gibberish resembling the sounds of a baby [syn: babble,
       babbling, lallation]