Hi Julie -
I'm mostly a lurker here, but I related so well to your post and just wanted to send you some hugs and support.
Our three children are about the same ages. I, too, have a 4 year old son and an infant daughter (4 months old today). We lost our second son, Elijah when he was 10 days old in 2007. He would have been two last August.
I'm not sure how your Jonah died, but you mentioned your husband finding him so I'm guessing it was a bit different than how we lost Elijah (born healthy, but developed respiratory problems, reason unknown). The fears I think, are similar.
I can't tell you how often I have woken up both my children from a sound sleep screaming for my husband and in tears because they didn't awaken right away or respond to my voice. For me the first two months were really stressful. I never slept, because when she was asleep I just stared at her to make sure she was breathing. My 4 year old is back to sleeping in his own room, but I check on him at least a dozen times a night. I don't know when I'll let my daughter start sleeping in her own room, unless I start sleeping there too!
Be gentle with yourself today, especially. Although Isaac is just fine, the experience last night was terrifying for you and your whole family. Are you home alone with your kids today? Is there someone (you trust) who can come over and give you a hand so you can just go in another room alone and have a good cry?
I'll be praying for you today that God brings you some peace and calm.
Gina