Hi all! First time poster

I suddenly realized that my FI and I (together for two and a half years, engaged for a few weeks) are planning to tell his 3-year-old son that Daddy and I are getting married, and we have no idea how to do that. Unfortunately, tomorrow is the day that makes the most sense to tell him, so a gradual introduction to the idea (if that exists) is out. We really should have planned this better! We also wanted to know if anyone had any experiences around telling the co-parent about remarriage/upcoming marriage- or how other people have told you, as the co-parent, that a step-parent is in the works.
Backstory- sorry it's confusing and complicated: FI and Mommy were engaged, never married, and broke up before the birth of their son. Mommy was engaged again last summer, but they'd only known each other for a few months and it ended weeks later- BUT 2nd fiance and his three kids moved in with her and her son.
Mommy and 2nd fiance broke up, and he disappeared off the face of the earth. He didn't even say goodbye, but just before leaving he fathered FI's son's half-sister (Mommy's second child), born recently. This wouldn't be relevant, except that FI's son spent a lot of time looking for former-future-stepdad (and former-future-stepsiblings) and asking why they'd left him, so I'm afraid that he might have some issues around the idea of "engaged."
I've known FI's son since he was one. We've always gotten along and liked each other- sounds silly, but true

I was introduced as a friend, and it was gradually explained that his daddy and I love each other very much (and love him very much too!) so I think we've formed a good foundation.
Unfortunately, Mommy has stated in the past few months that she feels that I'm "too confusing" to be around their son, and I've had to stay away a lot recently. I understand why she feels that way, but I don't think it's an appropriate thing to actually say, and it's just not fair to anyone, her son included.
It would take pages to describe, but I have been so respectful of her feelings and I have NEVER portrayed myself as FI's son's mom or even stepmom- more of a babysitter who's extra affectionate with his Daddy and says "I love you" before he gets in the car to go back to Mommy's house.
FI and I really don't want to have Mommy tell their son, but she will probably refuse to let him go with FI if she knows he's planning to tell their son about the upcoming marriage. Is it sneaky to tell her after we tell their son? Is sneaky wrong in this case?
There is currently no visitation/custody agreement (changing soon) so it's a concern that he'll be cut off from his child unless this is timed right- and in recent months she's refused to let their son go with Daddy, just because she thought I MIGHT be there.
I want to get this done, so I can go back to being overjoyed at marrying the love of my life, and stop being terrified a friend or relative will let it slip on Facebook and blow this whole thing up.
Any advice on how to tell a three-year-old about (re)marriage? Especially one who might have some trauma around the subject? Or how to tell an ex that it's not a threat to her status as a parent, but that Daddy's getting remarried?
...Or that maybe it's time to stop trying to keep me away, and time to accept that I am genuinely not a threat to her and only trying to build a strong relationship with a great kid whose life I'll be involved in?