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Reverting back to babyish behavior like whining, crying, baby talk, and having trouble separating is a natural reaction to change. Many normally well-behaved children are reluctant to break the rules by acting out aggressively (hitting and kicking), so they vent their frustrations this way instead.
Your son's behavior is likely related to some transition going on in the house, either with his own development or with his younger sibling. Now that the baby is a little older, maybe her newfound mobility -- if she's walking or crawling over the place -- has upset him. After all, she's probably starting to invade his territory and stake her claim to toys that used to be exclusively his.
Potty training, which often begins around this age, is also a big regression trigger. For many kids giving up diapers signifies the last piece of their babyhood and they realize that now more is expected of them. Your son could be responding to that pressure by trying to remind you that he's still a baby. If toilet training isn't going so smoothly right now, back off for a while, since he may not be ready just yet.
Even though this behavior is frustrating (especially when you're trying to leave a kid at daycare), be patient -- these phases usually pass quickly. Tell your older child often how much you love him and how proud you are of everything he can do. It's also important to hug and cuddle him a lot, so he understands that growing up doesn't mean he has to give up his parents' affection.
The answers from our experts are for educational purposes only. Please always refer to your child's pediatrician and mental health expert for more in-depth advice.