Why do children tend to choose the last option in a two-alternative forced choice task? If I ask my toddler if she wants bananas or apples, she will consistently choose apples. The same goes for colors, other related objects, or even seemingly unrelated items - She always chooses last.

Why is she biased to choose the last option? Is this bias specific to children, or is it a common phenomenon in adults too?

Is it related to Modality effect?
In general, and not specifically related to children, choosing out of a set of options often depends on people’s memory. In an ideal world, people’s options would be presented simultaneously, but in reality, options are often presented serially over time. Both the delay of time as well as interference from other cognitively demanding tasks are likely to increase uncertainty in one’s evaluations of more distant options compared to more recent options, and therefore increases the amount of regression to an overall category mean. In other words, imperfect recall introduces uncertainty in earlier options and judgments of the earlier presented options regresses as memory decays over time. Relatively desirable options will therefore tend to seem less desirable with time, and relatively undesirable options will tend to seem less undesirable with time. 

People therefore tend to select the first option in a set when choosing between generally undesirable options, and will tend to select the last when choosing between generally desirable options (Li &amp; Apley, 2009). 

In terms of kids I can add that their working memory span is relatively short, in the order of 10 seconds for a 1-year old (Garon et al., 2008), probably adding to the imperfect recall hypothesis stated above. Further, the executive functioning of kids is limited; most kids between 3 and 6 years are able to recall 4 digits or less in a digit span test (Espy &amp; Bull, 2005), further complicating a seemingly simple task for small children, namely recalling the options, let alone choosing a preferred option between them.    

References
- Espy &amp; Bull, Developmental Neuropsychology (2005); 28(2): 669–88
- Garon et al., Psychological Bulletin (2008); 134(1): 31–60
- Li &amp; Apley, J Behav Dec Making (2009); 22: 378–89