E.M.
Hi N.-
Hmmm, that's a tough one. Here are my thoughts. The first is a sound machine for your son- things like rain, the rain forest, ocean waves....even those world flute cd's you can get from Target for $9.99, and let them cycle continuously throughout the night. Plus, it helps with night sounds and any fears- they won't thing a noise here or there is out of the ordinary. Actually, you could use it for both- and the sound may distract them from that pure silence and they won't wake up as easily when one or the other makes a noise. The other idea is to create a sound barrier between the 2; split the room into 2 halves with either a book case or screen (you can hang blankets over it for aesthetic appeal). It should cut down on what they hear from each other- plus if they each have the sound machine or some music (some is made specifically to help deep sleep), it might keep them both in sleep land a little longer. I would put your daughter closer to the door so that she is easier access being so much younger. I hope this helps!
-E. M.