No matter how excited your child may be about getting a new sibling, that doesn't change an important factor: adding a child to the household is a big, big change, and sometimes young children need some help processing that. It's more than just an extra person to get to know, it's also acknowledging that they don't have sole ownership of your attention anymore, and trying to figure out how they fit into the new family dynamic. Here are some tips on how to help make that adjustment a little easier for them.
Share Your Time
This is a tough one, because with a newborn your time is stretched thin already, but it's essential. Your child needs to know that you're still there for them, and one of the best ways to do that is to make sure you set aside some time to spend only with them. This can't be time shared with a spouse or the baby -- this is one-on-one time just for you and them. It doesn't have to be a whole planned afternoon; just ten minutes per day, snuggling on the couch or playing a game, can make a huge difference.
Making time for your child reassures them that you're still around for them and that they're still important in your life. While these things may seem obvious to you, this is a time of uncertainty for your child and the reassurance can make a world of difference to them.
With the changes your family is experiencing, it's inevitable that your child will react to those, often by acting out. They may regress in progress they've made in various parts of their behavior, contradict your instructions, act aggressively toward the new baby, and just act out in general. It is important that you recognize the child is not planning all this out -- these are all just reactions to the confusion they feel as they adjust to their "new normal".
While it may be difficult to remain patient while parenting both siblings, one of which is an infant that requires a lot of attention and the other being an older child who is acting in ways that baffle you, try to resist lashing out or snapping at the older child. Instead, recognize that this is the only way they know how to communicate that they need you, and empathize with his confusion and vulnerability.
Pick Your Battles
Unfortunately, this acting out often manifests in ways that will affect your day-to-day life. Your older child may refuse to put their shoes on, or insist the crying baby leave the room. They might refuse to do their homework, or pitch a fit at the grocery store. Like a lot of parenting decisions, this comes back to one of the basics: pick your battles. If your child's defiance is manifesting as refusing to eat their carrots, it might be worth letting that go so that both of you are less frustrated later when you need them to put shoes on so you can leave the house. Prioritize, and try to avoid constantly fussing so your child (and you) don't feel too run-down.