Are you putting them down at the same time? If you are you should try to let your daughter stay up a little later or start putting your son down a little earlier. When our kids were sharing a room we put our son down first and let our daughter either lay on the couch or in our room until she fell asleep then we put her in their room so we did not have the problem of them waking eachother up. It worked for us but it really depends on how you feel about doing it, we would turn off the TV and lights and rub her back and tummy for about 10-15 minutes and she would be asleep, then we would wait about 10-15 minutes before putting her to bed, just to make sure she was in a good deep sleep.