The Rambam (Hilchos Mila 1:7) paskens that both a ger who had a mila as a non-Jew and a child born with a mila rquires hatafas dam bris. The m'kor for this halacha seems to be a gemara in Shabbos DAf 135 which says that the koton born with a mila rquires hatafas dam bris because of a safeik orlah kevushah-we are afraid the orlah is hidden and teh child is really an oreil.
There are a number of questions asked on this Rambam.
1) The Minchas Chinuch (Mitzvah 283) says that the Rambam paskens in Hil Teruma that a chikd born with a mila can eat terumah. The question is why. If we are afraid that he is really an oreil, so we should be machmir m'safeik and not let him eat terumah.
2) The Rambam in Hil Mila Perek 3:6 writes that both the ger and koton do not make a beracha on the hatafas dam bris. I can understand that we don't make a beracha on the koton since the whole reason for the hatafas dam bris is due to a safeik orlah and m'safeik we don't make a beracha. However, the reason for the ger can't be because of safeik orlah kevusha-the ger had a real bris. The only reason he needs hatafas dam bris is because the milah was done when he was a non Jew. Why wouldn't he require a beracha?
3) The Kehillas Yaakov in Shabbos asks that the gemara in Yevomos 71a learns that a father who has a katan sh'nolad mohel can't eat the korbon pesach (until the hatafas dam bris is done). Why is this different than teruma where we say the katan can eat terumah.
4) The Minchas Chinuch also asks, if we are choshesh for orlah kavusha, how does hatafas dam bris solve this problem. All you are doing is drawing some blood-you are not cutting anything away.
There are a few mehalchim to answer these questions. Below is the mehalach of the Mishkanos Yaakov (Y.D. Siman 63)
He writes that b'emes there is a machlokes hasugyas between the genara in Yevomos 71 and Shabbos 135. The gemara in Yevomos brings the shitta of Rabbi Akiva that we learn from a posuk that both a ger who had a mila as a non Jew and a father who has a katan sh'nolad mohel can't eat the korbon pesach. If the reason why the koton needs hatafas dam bris is because of safeik orlah kevushah, why do we need a limud for this? It is pashut-the child is a safeik oreil and m'meilah you can't eat the korbon Pesach. Therefore, you have to say that according to Rabbi Akiva, the reason for hatafas dam bris isn't because of safeik orlah kevusha but rather it is a din in the mitzvas mila-part of the mitzva is a requirement to do hatafas dam bris. In fact the zohar says there are 3 parts to mila, the mila, p'riah and hatafas dam.
The gemara in Shabbos argues on Rabbi Akiva and holds the reason for a katan is because of orlah kevusha.
Furthermore, According to the gemara in Yevamos, the reason both a ger and a koton need hatafas dam bris is the same reason-both are missing the mitzvah of hatafah of dam bris. The ger had a ma'aseh mila but there was no hatafas dam l'shem bris.
We can now say that the Rambam paskens like the sugyah in Yevomos and hatafas dam bris is itself a mitzvah . (unlike the Rif and Rosh who hold the reason is orlah kevusha). We can answer the questions above.
1) The issur of eating teruma is only for an oreil. Even though the koton is lackning the mitzvah of hatafas dam bris, he is still not considered an oreil. (ayin R' Chaim al hashas that says the same idea). Since he is not an oreil he can eat terumah. [L'chorah you have to say that the Mishkanos Yaakov understands that the shem oreil goe saway when the orlah is removed or is not present. Since this koton does not have an orlah he is not considered an oreil even though he is missing a chelek of the ma'aseh mitzva of mila (i.e. the hatafas dam).
2) The reason we don't make a beracha has nothing to do with safeik berachos but it is because although the hatafa is a chelek of the mila, the chachamim were not misakein a beracha on the hatafa by itself. Furthermore, the Mishkanos Yaakov points out, in Mila 3:6 the Rambam mentions an androgonus doesn't make a beracha on hatafas since it is a safeik beracha. We see that koton and ger must be a different reason.
3) The Kehillas Yaakov says that the issur of bringing the korbon pesach does not depend on the shem oreil, but rather on whether the mitzva of mila was completed. The proof is that you can't bring a korbon pesach if your slave doesn't have a mila-even though you are not an oreil. Therefore, since this koton still requires hatafas dam bris the mitzvah of mila has not been completed and you can't bring the korbon pesach.
4) Since the hatafah isn't there to remove a safeik, there is no need to do any cutting of teh skin, and letting out blood is enough.
Other achronim (Imrei Moshe, Mishnas Ya'avetz) point out that the Yerushalmi seems to work well with the sevara of the Mishkanos Yaakov and thsi coulds also be the m,'kor for the Rambam.
Both the Kehillas Yaakov and Mishnas Ya'avetz don't like the fact that the Mishkanos Yaakov made this into a machlokes hasugyos between the gemara in Shabbos and Yevamos. They both have a mehalech which puts the sugyos together, but ain kan mokom l'harich. ayin sham.