I think the idea about giving them only the info they really need and keeping it strictly to details about how it will affect their lives- is good advice.
The day I moved out of Ex's house my 5 yo ds had a sleepover at his cousin's. The next evening I picked him up and we went out to dinner and I told him we were going to sleep at Grammy's house that night. He asked why and I told him that I was not going to live at Daddy's house anymore so I'd be staying at Grammy's until I could get my own house. He said- And I'm going to sleep there with you? And I said yes and sometimes you'll sleep at Daddy's. That was it.
Ex and I had discussed how to do it. Ex wanted to be there when ds heard the news. But he really only wanted to be there to be sure I didn't say "we" decided to live apart. It was not a mutual decision and he was more concerned with not taking any responsibility for the break up than anything else. He was very angry at the time and I was concerned that things might turn ugly. I definitely did not want ds to witness us in a battle right there with im as captive audience. So I told Ex that I was telling him. It turned out fine.
He did ask questions over the next 2 months or so like- why don't you want to live with daddy?, will we ever live at daddy's house again?, can daddy come on vacation with us?, let's call daddy and see if he wants to come play mini golf with us, etc. And every time I would simply address the question gently with something along the lines of- me and daddy aren't together anymore, daddy's with Jamie now and they will go on vacation together, or daddy's working and can't play mini golf with us.
Personally- I would avoid breaking the news to my kids so close to a special family day like a birthday- especially one of their birthdays- if at all possible. They will forever have that association. Since your STBX hasn't lived with you in so long anyway and nothing much is changing for them right away- I would think you could wait a week or two to tell them.
Good luck- I know I dreaded telling him but it turned out when I did it was a huge weight off and he was minimally affected by the actual news. It was more of a gradual adjustment to the changes afterwards.