The Rambam writes in Hil Aveilus 3:5:
המטמא את הכהן. אם היו שניהם מזידין הרי הכהן לוקה וזה שטמאו עובר על ולפני עור לא תתן מכשול. היה הכהן שוגג וזה שטמאו מזיד הרי זה שטמאו לוקה:
It's not clear in the first case, where someone is m'tamei a kohen, what wrong action the kohen did that would lead to his being chayav malkos, but even harder to understand is the second case, where the kohen is shogeg and someone is intentionally m'tamei him. Why does the m'tamei get malkos? There is lifnei iveir in causing someone to sin (like in the first case), but why more than that?
There is a parallel Rambam at the end of Kilayim 10:31:
המלביש את חבירו כלאים אם היה הלובש מזיד הלובש לוקה והמלביש עובר משום ולפני עור לא תתן מכשול. ואם לא ידע הלובש שהבגד הוא כלאים והיה המלביש מזיד המלביש לוקה והלובש פטור
And similar questions are raised there by the Kesef Mishne.
Since it relates to our parsha I want to point out a second difficulty with the Rambam. Rashi tells us at the beginning of our parsha tells us that the double-language of "amor... v'amarta" teaches "l'hazhir gedolim al ha'ketanim." The din (see Yevamos 114) is that adults are not obligated to stop children from doing issurim - "katan ochel neveilos ain beis din metzuvim l'hafrisho." A father may have to stop a child from sinning as a function of chinuch, but that's between father and child and no one else. The Tur writes that our parsha is an exception to the rule -- "l'hazhir gedolim" means adults have to intervene if the a child kohen is going to be m'tamei himself. The simple pshat in the gemara, and this is the Rambam's position, is not like the Tur. "L'hazhir gedolim" does not mean that an adult has to intervene if the child chooses to be m'tamei himself. What it means is that there is an issur of an adult being m'tamei a child, even though the child is not chayav anything.
Minchas Chinuch and others ask: why do we need a special derasha to teach us this issur to be m'tamei a child? The Rambam himself, as we saw above, holds that someone who is m'tamei a kohen is chayav malkos where the kohen himself is shogeg. What's the difference between being m'atmei a kohen who is shogeg and therefore not liable himself and being m'tamei a kohen who is a katan and therefore not liable? If the m'tamei is chayav malkos in the first case, isn't the second case obviously true by extension?
I barely have time to write up the questions, so don't hold your breath for a post with answers : (