Well poo! I typed a long response and my server ate it.
I just watched a ton of birth videos at a four day Bradley workshop. I think most or all of them were produced by the AAHCC. A good one for easing kids into it might be 'Birthing with Joy'. It is five births, all natural. It has no full on vaginal shots which might help them get comfortable with the whole thing. The other great one is called 'Children at Birth' and shows three or four births that are attended by older siblings of various ages.
Another thing you might try is taping the dreaded Baby Story until you have a couple of nice homebirths... they do run them now and then! I think you do need other videos too, though because they never show the actual birth and you don't want the kids to be surprised by that on the big day. There is a listing of Baby Story
lists episodes by the last name of the couple and if you click on the name there is a brief description, so you might be able to narrow it down to homebirths for your taping.
Good luck! What you are doing is so great!