I'm cosleeping with my 7 week old and my toddler and ever since the baby arrived, my toddler has increased his need to nurse at night. Like your toddler, mine can go to sleep without nursing if I'm not there, but of course, I am always here. Before the baby came, I started singing the alphabet song as a cue that it was time to finish nursing. He objected to limits on his nursing at first, and always asked for more milk when the song was over, but he gradually accepted it. Now when I nurse him down, I don't have to lie there for an hour; I sing the alphabet song when I feel he is relaxed enough to go to sleep, then hold him for a few minutes until he drifts off. This works only because my dh is holding the baby during this nightly ritual. My toddler gets me all to himself and gets to be the baby during this time.