When children regress
Its amazing. Your child is becoming more responsible, mature, following instructions faster and is blossom before your eyes. You're bragging to friends and family about how smooth things have been in your household and that everything is just going so well.
Then, out of nowhere, some one turns back the clock and you have a baby on your hands again. You're hit with waves of whining, crying, screaming, and even wanting to be held all of the time. Your once independent child is now insistent on you dressing him and has suddenly forgotten which shoe goes on which foot. They begin to thumb suck, want to sleep in your bed, and wont leave your side.
And the baby talk.
One of the most common behavioral shifts during a regressive period for a child. They begin to leave out words in sentences, make their voice higher pitched, and seem unable to communicate any desire in a sensible way.
When working with parents, they often share that baby talk is not something they know how to manage. Mainly, it comes from a concern with how the public will view the child and share that they feel embarrassed with how their child is acting. Caregivers are often mystified why their child has been regressing and even more frustrated that their once clear communicator is babbling like a two year old again.
Reasons why children regress and what to do about it
1. One of the most common reasons why a child regresses is a new sibling. Kids LOVE attention and when they see that a new baby is in the picture, they try just about anything to regain that #1 spot back in a parents eye. Their feelings are out of whack and they see that a new baby gets plenty of attention when they cry, roll over, throw food on the ground, or need a new diaper. So obviously your 5 year old is going to start doing those same things to try to get the same response from you! If its working for baby, it should work for him, right??

Many parents natural response is to try to shut that behavior down as quickly as possible. They say things like:
"You're a big boy, not a baby"
"You know better than to do that"
"That's for babies"
They are well intentioned comments coming from a place of guidance, however, they often are laced with shame which the child picks up on without fail. It frequently leads to anger and upset from the child and can drive resentment between the siblings. Plus it will most likely continue to happen despite what the parent says. Kids are fun.
What do you do then? The best bet is to provide an outlet for the behavior within some framework of limitations that fit for you. When our son began to baby talk after our daughter was born, I allowed him to do it within the confines of our home but not out in public. He could pretend to be a baby as much as he wanted while we were home including wearing onesies, pretending to get a diaper change (he was fully potty trained), get swaddled like a "burrito baby" as he put it, and even put a paci in his mouth. But as soon as we stepped out of the door, he had to be is 3 year old self. This worked well because I allowed him to get any needs met that he was trying to express while still fitting within my boundaries of not wanting deal with two babies in public. Find where your limit of comfort is and create a space for it.
Another sure bet is making 1:1 time with your child away from the other sibling. Everyone likes to feel special and have full undivided attention, including kids. You can take him/her to the movies, on a bike ride, or play a game. As long as it is peaking their interest and carving out quality time, they are less likely to try to gain your attention in other ways. Here are some sure fire ways to bond with your child (clink each item to see link):
Jenga
Play the original way or write question prompts on each block for conversation starters
Toss and catch
Fun and easy for any age and developmental level. Use inside or outside. Great way to chat about any topic while playing
Building set
Let the child lead while you build and create together
Science Experiment book
Have a kiddo who loves to learn and experiment? Choose a page in the book and let the exploration begin

2. Another reason children can regress is a new stressor. This can be as obvious to adults such as:
Divorce
Death of a loved one
Parental incarceration or deportation
Or less obvious ones such as:
Abuse (when the parent is unaware)