According to Rashi's first commentary on Vayikra 25:46, what would be the distinct meaning of the verse with "והנחלתם" vs. with "והתנחלתם"?
והתנחלתם אתם לבניכם means, you may hold them as your possession, לבניכם אחריכם, for the benefit of your children after you. It would not, however, be correct to give the word והתנחלתם the meaning: “leave them as an inheritance to your children”, for if this were intended it should have written: והנחלתם אותם לבניכם (the Hiphil), but והתנחלתם (which is the Hithpael form) is equivalent to והתחזקתם “you may hold them as your possession”.
In Hebrew, the word נחלה means inheritance. If the word would have been written והנחלתם it would mean to pass on the inheritance, or bequeath. Since the verse says והתנחלתם instead, the verb is reflexive and refers to an effect on the one doing the action (hispael in dikduk terms). In this case, the command is to the parents to keep the slaves as a potential inheritance for their children later on.