Oh, god OP, my heart goes out to you and your DC. 
My OH died suddenly 9 years ago - here's some thoughts on what would have eased some of the burden on those first unspeakable few days:
The MN bereavement forum is a godsend. There will be empathetic people on it all through the night as, like you, they will be up at strange hours.
Cry. Don't even try to hold it together when you need to howl. It hurts so much that you feel like the pain is cutting you in two. Giving into the crying, will help bring a release for those many moments of excruciating pain.
Get someone else to do your shopping. Not just because you won't be thinking straight, but because shop assistants will ask you 'how are you today' and you won't know how to answer. Best to avoid it if you can - believe me.
A previous poster said get someone to deal with the flowers -YY to this. The last thing you need around you is a house full of wilting flowers.
Winston's Wish is an incredible charity as PP have said. Child Bereavement UK was also a life line to me, along with Widowed and Young (sorry, that name will seem harsh and uncomfortable at first
).
This first week will probably be the hardest thing you've ever done. Get through it two minutes at a time. I remember alternating between desperately wanting people to tell me we'd be OK, and then feeling really angry with them if they told me we'd be OK.
A huge, huge, huge, huge hug to you and yours, OP.