So where is the girl going to be?
you said that house has 3 bedrooms... I assume one for you/Hubby, and 2 others? Plus that bonus room?
I would let each child have their own room.
As the previous poster said, I would NOT make one of the boy's room a combo of it being a bedroom/play room for them all. That is NOT fair. NOT fair at all.
Also, as the kids grow up, once in elementary school, they should all ideally have a desk in their rooms to study etc., and thereby learning independence and responsibility too. AND, you/Hubby have our own desk where you want.
I grew up, with my own room and so did my siblings. At one point we did share, and it was NOT FUN AT ALL. Also, at a certain age, a kid just needs "privacy" and "autonomy" in their rooms... and to decorate it themselves and have their own tastes and color choices. I think, that is all part of growing up... AND being allowed to have your own tastes and learning how to discern that.
If you REALLY want an "office"... then why can't you/Hubby make an addition to the house? Or convert a part of the garage??? many people do that.
This isn't about "winning" over your Husband. But thinking about what would be best, for your kids. What if when you grew up, sharing a room even if you wanted your own privacy, and having your own room WAS possible given your home's layout... but it was denied to you because it was your parent(s) that did not want you to have your own room, because they wanted to 'win' about the extra room being an office. Though nice to have... a growing kid... can outgrow a room that is "shared." Then what? An "office" if you want it that bad... can be had... but by doing an addition, or converting the garage or converting a closet space... or building in a "closet" space somewhere else in the house. Try finding an area/wall that you can "claim" to make an office and put a desk etc. Or get an Armoire... and convert that.
all the best,
Susan