Well.. If you stick it in, even 2 inches, she is no longer a virgin and it could break her hymen. Make sure you both understand that. If she's not ready to lose her virginity and have full intercourse, keep it in your pants. Definitely talk to her about all of this first. A girl will never forget her first time when it comes to losing her virginity, so make sure she is 200% ready and don't just think of yourself. (not saying you will).
But even sticking it in a little bit as you say, pre cum can get her pregnant. Even if you don't stick it in, but rub it around her area, she can get pregnant from that too. Any semen around that area can get her pregnant. So if you're going to do that,
wear a condom. You should
always wear a condom anyway. If you're too young or not ready for a child, wear a condom.