Sleep is a very important factor in every child's development. Sleep can change, sleep patterns can change and the number of hours kids can sleep at night and their nap schedules changes from their younger years to their later years.
You might find that by the age of two or so, and three, all of a sudden your child, who was going down to sleep very nicely, might now not be. They might be coming out of their bed, especially if you moved your child from perhaps a crib to a toddler bed, and now they have the independence feeling where they can climb out of bed on their own, and make frequent trips to mommy and daddy's room. That's very common.
The best approach is to let them come to your bedroom. I mean, you can't stop them. But you can certainly guide them back to their room and give them some reassuring words and tell them good night and maybe tell them 'OK. It's time to sleep. You sleep in your room. Not in mommy and daddy's room.' And you might have to do this every night until they're starting to get that.
Another common sleeping issue that occurs around the age of four is all of a sudden your four-year-old, who was going to sleep nicely, you put him to bed and you see them in the morning, is now waking up in the middle of the night and fearful of monsters. This is very common and natural and would require you to come into their room and just reassure them that all is OK. There are no monsters. It's time to go to sleep. Hopefully you'll only have to do that several times before they'll get that, although you never know with children.
These are some of the common sleeping issues that you'll find and what's best to remember is that your child may wake up in the middle of the night. They may need you. When you're potty training, they may wake up needing to go to the bathroom, or having an accident in the middle of the night. These are all common sleeping issues that occur and that you should expect during those formative years.