Put the child back in timeout and reset the time. Without exception. Like most other parenting duties this is all about consistency.
Ignore laughing or other such - the child is trying that behavior out to elicit a reaction from you. Stay calm and firm. Don't interact beyond enforcement (that downtime's the point of the time-out, and they may be trying to pass the time in a more entertaining manner with you!).
At that age, timeouts don't work unsupervised, you have to be right there. It can be a hassle, but that's parenting.