Depending on when this started may tell you what might be going on. If this is very recent it could be due to their developing mind into the world of imagination. This usually starts around 2. They might be scared. If this is the case, there are products you can buy to help with this. If a situation has changed recently that might be another clue. Develop a bedtime routine. Limiting their exposure to tv and brightly lit rooms an hour before bedtime can help their body tell them it's time to go to sleep. It's much easier to put a sleepy child to bed than one who is wide awake. Try changing up the bedtime ritual. Have them brush their teeth and do all of the others things that normally are done right at bedtime. Have them do all of these things about an hour before you want them to go to bed. After all of this is done, it gives them an hour to settle down. Try and have them in a dimly lit area of the house this last hour, no tv,computers, or anything that is giving off artificial bright lights. their body should start to send the signals to sleep. What ever you do, I would recommend that you not start letting them crawl into bed with you. You need to Calmly walk them back to their bed and say good night. No cuddles or anything of that sort. You will just be rewarding the behavior. You'll get tired of this, but hang in their, they'll eventually get it.