Final answer:
Rachael is still very young at two-and-a-half months and not expected to have established routines, which is normal for her age. Brianna, at 18 months old and showing frequent crying and difficulty in soothing, might be considered to have a difficult temperament by Thomas and Chess. One-year-olds generally can follow simple commands and show separation anxiety, which is part of normal development.
Step-by-step explanation:
The developmental milestones of infants vary widely and reach beyond simple physical growth to include sensory, motor, social, and cognitive developments. Rachael, at two-and-a-half-months old, is in the very early stages of developing routines related to eating and sleeping, but because of the typical infant development stage, routines may not yet be established. In contrast, Brianna, being 18 months old, is expected to have more developed patterns and behaviors.
Thomas and Chess might categorize her frequent crying and difficulty in soothing as possibly indicative of a difficult temperament. It's important to note that, at that age, infants like Brianna may demonstrate separation anxiety and strong attachment to their parents, which may cause them to wake frequently during the night and be hard to soothe. In general, by the time infants are around one-year-old, they can follow simple commands, mimic actions like playing peekaboo, and experience distress when separated from their caregivers. In the case of Rachael, since she is much younger, comparing her to children several months older may not provide a conclusive temperament classification yet.