H.G.
Wait a little longer before getting married. (I know that must be so hard- you've found such a wonderful partner!) And YES get counseling! Family counseling, with someone you really like and trust.
Don't push them beyond basic politeness. (Which it sounds like you aren't!) They are so full of hormones and insecurities right now, and when you add in their feelings about their father...it has got to be an absurdly hard time for them! They may be feeling shame about "getting over" their dad, and not remembering him as much as they think they should. And they may be very angry at you, for loving a new man.
But, in time, they will figure it all out. With counseling help, this will happen much faster! And they are watching you search (and find) your own happiness in this world. That teaches them such a valuable lesson, though they don't know it yet. You will definitely turn their lives upside down by getting married, and your own as well, but that doesn't have to be a bad thing! A challenge? Yes. But challenges can be rewarding, when you make love and communication your top priorities.
Congratulations, on being such a strong woman and mom! You sound like a devoted and lovely person. It will work out!