No. You tell her it's a great compliment, and then walk away.
No 19 year old is ready for the pressures of having a wife and two children they did not father, nor should you *ever* go anywhere near kids to whom you are not willing to commit forever.
As a 21 year old mother, she is looking for someone to share the load, and you are not the one to do that, no matter how smart or earnest you might be. The human brain doesn't fully mature until the age of 25, and you both are going to be changing a lot before then.
She needs to act as a parent first, and a woman second, and not get involved with anyone who isn't actually ready to take on the job. You are nowhere near that, and this isn't just about two single people. Don't have any part of hurting those kids by getting involved, and then disappearing. She needs to look for someone old enough to handle the responsibility, and to stay around. That isn't going to be you.