They don't HAVE to get married!
I have a former sister-in-law who at age 22 married a man who was also 22 and already had a six year old daughter. Still very young, but they've had a long, happy marriage and have another daughter together who is grown now. He and his high school girlfriend were both sixteen when his first daughter was born. Cooler heads prevailed and everyone realized that a "marriage" between these two CHILDREN would be a disaster.
Instead, they settled into a relationship that resembled a very harmonious divorced couple, and it worked very well for them. He has ALWAYS been a father to this girl, and she was given his last name. He worked after school and paid child support since the day she was born when he was just sixteen. He had liberal visitation and spent lots of time with her, never missing the holidays, birthdays, school plays, etc. When he married my sister in law, she became a loving stepmom to this girl. She was very much a part of our family and the other kids considered her to be a cousin - they still do.
How much better was this than a hurried "marriage" doomed to failure between two kids who were ill prepared and in no way ready to make a lifelong commitment. They both went on to have long, happy marriages with other people and both have had more children with their spouses. So, it's not easy but it CAN be done.
And here's a thought ... how will your daughters strange new family feel if her baby, their anticipated "heir", turns out to be a GIRL????