It is important to ensure that the child is not sick or has a specific need.  Otherwise, if the toddler is well and this is occurring every night, the child has likely has developed a sleep association problem: she has learned to fall asleep only when her parents are beside her.  When we sleep, we naturally cycle through stages of sleep.  During the periods of lighter sleep, a child may awake, notice that the environment has changed (eg her parents are no longer beside her), become distressed and have difficulty falling back asleep.  These sleep-associations can occur with tv, a bottle, music, or with a parent.  The best advice is to cousel the parents about sleep hygiene, including a consistent bedtime routine, then leaving when the child is drowsy, but still awake.  The child will eventually learn to fall asleep on their own and can self soothe back to sleep if they wake up overnight.  It takes perseverance and patience!