Helping an older child adjust to a new baby being brought home can be difficult. I've got some tips for how you can acclimate your child to the new situation based on an article published in Psychology Today
First, inform your older child you are having a baby. Then, you have to tell them why because in a kid's mind, the first thought is, "What?! I'm not enough? You don't like me and are replacing me?" For example, you could say, "We decided to have another baby so you could have a brother or sister, and you will never be lonely," or you could tell them, "When you come home from kindergarten, you will have a little playmate." Even if their sibling won't be able to do much for a while, it's still something you can have them look forward to.
Second, tell your child some kind of success story. Say, "Mommy is such good friends with her brother, your Uncle George, and it's nice to have a brother and a sister. We want you to have that kind of fun relationship." So, you are setting something up for them that already exists that they can appreciate.
Next, reassure your child that love does not get subdivided. If you have a pizza and half the pizza goes to someone else, the child knows he or she is only getting half the pizza. That's a child's mind. You have to tell the child, "It's not like pizza or a cookie. Love grows. There's always more, more, more. There's love for Mommy, there's love for Daddy, there's love for you, there's love for aunts, uncles, grandmas, cousins, and the new baby. Mommy and Daddy have so much love you're not going to miss out on one shred of love."
Also, it's really important that you show your child his or her baby pictures. Show them when they were first born, when you had to feed them, when you had to bathe them, etc. Say, "See you couldn't do this yourself when you were a baby. Now, you're a big kid and can do it all. But at the time, Mommy and Daddy had to do it for you." This will help the older child understand he or she received the same kind of attention the new baby is going to get. Reassure your child that over time the baby's going to grow up just like them. He or she is going to be able to do things by him/herself and won't take up as much time. You have to remind yourself of that too. I had to remind myself of this too because I thought the rest of my life was going to be spent with a screaming kid. But kids go through phases, and this one will pass.
Another thing you can do is educate your child about babies. If you know a family with a new baby, bring your kid over there. You can show your child how tiny, fragile and dependent babies are. Show your child that everybody will need to be gentle. Point out how babies can get really annoying and cry, but they sleep a lot. Admit, "When the baby's first here, he or she is not going to be able to play your favorite games. You have to wait until he or she is older." But then, talk about the things they can do with the baby - take it for walks, sing to it, read to it, hold it, etc.
One of the things to always point out is that your child will have a special role as a brother or sister. Talk about how they will be able to teach their brother or sister the alphabet, counting, writing, and riding tricycles. Explain how the baby's going to look up to them as a brother or sister because they already know so many amazing things.
It's also really important to talk about their emotions. The truth is sometimes they are going to feel left out, angry, and annoyed because they want the attention, and the baby is either getting it or just being noisy. These are all normal feelings. You have to acknowledge that they're normal. You can say, "Sweetie, when you feel like you need a hug, just come over. When I'm feeding the baby, you can cuddle with me, and I can read you a book while the baby's drinking the bottle or drinking from Momma. But sometimes sweetie, you will have to wait because it takes time to put the bottle together (or whatever it is you're doing) and babies can't wait. Big boys and girls can wait a little bit, but babies can't. So while the baby is a baby, there are going to be times where you are probably going to be a little annoyed. But you're a big kid and can do some things for yourself; the baby can't do anything." When you lay out what all the emotions are probably going to be, then kids don't feel ambivalence, guilt, anger, annoyance, and rage. They are also less likely to act out violently.
Lastly, make your older child feel involved. Tell them when the baby comes, it would be nice if they would pick out its clothes or bib. That way, they feel a sense of some responsibility. When you ask kids their opinion and give them some responsibility and power, it's amazing how they get less petty because they still feel important.