I kinda agree with Melody. I don't think your kids will necessarily mourn the "loss" of their father in the same way that you do. After all, they will have you right there, and they will be surrounded by loving parents.
If I made a video I might not make a specific video that was like a monologue for them to see later in life that was apologetic in tone, or sort of setting the expectation that they will see your transition as loss, but more just get some videos of you playing with them, and being yourself. It feels to me sort of like making a video monologue like that kinda sets the expectation and mood of loss of fatherhood. But transition doesn't feel that simple to me - because even though there is some loss, it's not a complete loss because you still are who you are. You still are their father, even after transition (even if they don't call you that).
I dunno, with me you kinda have to take my advice with a grain of salt. I'm not sure if it's helpful, but I wanted to offer up my thoughts on it. I think it's so cool that you are thinking about this sort of thing though, trying to capture some images of yourself before transition. It's really thoughtful, and beautiful that you are thinking this way.