I am sort of in the same boat. I was hoping that reading the responses you got would help me, but they haven't. I have been with my boyfriend for 1.5yrs (and just moved in with him). Although their father is still in the picture, my boyfriend has done more for the kids lately than he has. Their father has really started slacking lately (not taking them when he is scheduled to, not paying chid support), and my boyfriend has really stepped up in his absense. I tell him all the time how thankful i am for his help, and how wonderful it is to have someone who loves all of us that much. And, I want to do something special for him, but im not positive he would react the way i would hope. AND, im not sure my 6 six yr old would understand it either. She knows he's not her Dad, and I dont want her to feel wierd either. It's just an odd situation... I wish i knew how to handle it.
Amyhow, sorry i couldn't be of any help... just letting you kow you aren't the only one dealing with this.
Best of luck to you and your kiddo.