many sympathies - DD only started sleeping through (till 5am) at 10mths, and has just these last two months gone longer in the morning. But she did go down in her cot from 3mths. Personally I would try really hard to get her in the cot as co-sleeping drives me nuts but appreciate that may not be easy.
Re your DS - I get DS to stay in his room while I give DD her final feed and what I say to him is, if you stay in your room, and don't come up, I will give you an extra story in my bed ALONE while DD goes to sleep (they share the room). And then we have a special cuddle and then I herd him back to the bedroom with dire threats of being quiet and setting a good example to DD. Could you try someting similar? Small reward for staying put? If DS can't stand it anymore, he does come up but I make him sit on the stairs on the landing and he doesn't get the story.