Should I let them find him?

When I was 16 my boyfriend of 2 years and I had for the first time, but half way through I wanted to stop and he wouldn't. I wasn't stupid enough to stay in that relationship, so I left him. I told my parents what happened and they were very supportive. Then my ex started calling all the time begging me to come back to him but I wouldn't budge, it started to annoy my parents when he began calling at 3am. A month later I found out I was pregnant, I told my mum and she told me that she was pregnant too. We decided to move down to Melbourne, we all thought a change of scenery would be good, I never told my ex I was pregnant. When we moved into our new house, I decided to leave school so I can look after my baby, 8 months later I had the twins Sienna and Bradley. Sienna and Brad are now 14 years old and they told me yesterday they wanted to find there father, I didn't tell them how they were conceived, I don't think they would understand, I think I overreacted a little, do you think I should tell them the truth?, do you think they are old enough to understand?, should I let them find there dad?