Can a boy, who has not hit puberty and therefore has not produced semen, get a girl pregnant just by pre-cum? Can a girl get pregnant just by pre-cum when do you start to produce semen?
Before I get to your questions, I need to point out that regardless of whether a boy is able to produce semen or not, having your penis in a girl is still fornication (I Corinthians 6:9-10).
If a boy has not yet reached puberty, he is unable to produce pre-ejaculate fluid (pre-cum).
Pre-ejaculate fluid is produced by the Cowper’s glands. The seminal vesicles produce semen. These two sets of glands both develop during adolescence, but typically one set matures before the other.
If the Cowper’s gland matures first, then a boy drips pre-ejaculate fluid when aroused. This fluid does not contain sperm (which comes from the testicles). However, eventually, the seminal vesicles will mature and when a boy is strongly aroused, semen is loaded into the ejaculatory duct along with sperm from the vas deferens. Depending on the boy and the circumstances, it is possible for some of that semen to drip into the urethra (the tube in the penis). The pre-ejaculate fluid would then mix with it and push the bit of semen out of the penis. Thus, it is possible for a boy to get a girl pregnant without actually ejaculating in her vagina. The odds are low, but not zero. See Can pre-ejaculate fluid impregnate a girl?
One problem boys don’t consider is that you don’t know when your seminal vesicles will mature. Thus, you don’t know when sperm will first be present in your pre-ejaculate fluid.