I have a half-sister that had a really hard time with feeling like she belonged anywhere once I came along. Blended families were not as common then so my Mom did the best she could (weren't any real resources to go to for advice). But in hindsight, it has really affected my sister as an older child and now adult. The good part is that your kids have each other, so no matter what happens (despite good intentions, emotions and insecurities always muck up the picture) there is a sibling that is 100% theirs. But this is what my sister said:
"I think having a shared title for “Dad” is a really good thing. And YES…it would have made a difference. And while I LOVE the idea of another language, it shouldn’t be another language for “Dad” because, they are saying the exact word they are trying to get around. But, if the older kids don’t mind it so much, then I say go for it. The baby should be able to call its father, daddy, just like you are trying to respect the older kids right (plus, why take that pleasure away from your husband). However, if the older kids are feeling uncomfortable with the “Dad” other language thing, then I would suggest a nickname…it should be something for the older kids to call him, which makes them feel special because “they” have something to call him that only the family might understand…it should be something the baby can call him if he wants to, but they should not take away “Dad” from the baby when he/she is old enough to understand what's going on. The baby would just call it both names when it's older. This is an identity thing and when kids are in grade school…it’s all about having a “Dad” and “Mom” and if he/she is calling “Dad” something other than that, it could cause turmoil later in life, which the parents probably wouldn’t know about nor really understand.
So…after all of that (I was thinking as I was writing), I would suggest the nickname. Something special for the older kids, something the baby can use until he/she can understand the difference and something that makes “Dad” and “Mom” comfortable. Just don’t take the title “Dad” away from the baby…
Idea for nickname: Have each child list words that best describe their Step-Dad and then cross reference the words and then look those words up in another language. For example: Some friends of mine call each other “Shots” which is totally misspelled. But, it means “sweetheart” and for them…that is special."
And on a side note. My sister called my father by his name but I never got confused as to what I was supposed to call him. He was my Dad. But in reading the above note from you and then from my sister, it seems that your request is so the baby doesn't get confused and it really seems like it should be to honor the relationship he has with the older kids. Your baby is a sponge. It won't get confused. And by the way, I have a three year old and we have just taught our daughter our names (I work in events and have had to ask one too many lost kids what their parents name was only to have them say, "mom". - I can't ask for Mom to come to the show office and I hate announcing that there is a lost child - you never know what freak is listening). Anyway, my point is - every once in a while, she'll try and use our first name to be funny - we simply tell her that she calls us Mom and Dad and other people call us by our name. She get's it.
Anyway, hope this helps. Good luck!