The Posuk (דברים יא,יט) starts off:
וְלִמַּדְתֶּם אֹתָם: Teach THEM
In the plural. However, the Posuk ends:
וּבְשָׁכְבְּךָ וּבְקוּמֶךָ: When YOU lay down and get up
In the singular. Why the change?
On a related note with regards to proper chinuch: The Chasam Sofer's Rebbe (one of them), Rav Pinchas Halevi Horowitz (better known by the name of his magnum opus, Haflaah on meseches kesubos), has an interesting comment. The gemmara tells us that if a man is like an angel of G-d, one should learn Torah from him. In the hakdamah to his sefer, the Haflaah explains that an angel cannot walk (angels have one leg) he merely stands in place doing G-d's work. The nature of man is that he is a holech, he walks from one place to another, one task to another, one level to another. Namely, he is growth-oriented. When a teacher must teach material they have mastered to their students, they can no longer grow in their learning, they must stand their at a status quo doing G-d's work by teaching unknowledgeble students. The Gemara's intention, says the Hafalaah, is to tell us that the fitting teacher is one that gives up his ability to grow for himself, a holech, to be like an angel instead, standing in one place, not growing, in order to do G-d's work.
A teacher is one that gives up on their own wants and desires, even their G-d given right to be growth oriented, to do the holy work of וְלִמַּדְתֶּם אֹתָם.
The Chasam Sofer answers this is a lesson in Educating Children.There are Parents who are always telling their children the importance of learning, but they themselves are busy with everything but learning. The lesson is if you want your children to learn meaning you want to teach them plural your children then you must be Singular:וּבְשָׁכְבְּךָ וּבְקוּמֶךָ You singular learn in your own house serve as an example. In short do not says do as I say and not as I do.
Actually, YS asked the question using an imprecise translation of "וְלִמַּדְתֶּם אֹתָם". It really translates as "Y'all shall teach them (meaning the words of the Torah) with your sons". The Hasam Sofer's question must be on the plural in the word "וְלִמַּדְתֶּם " which does not fit with the singlar form of "וּבְשָׁכְבְּךָ וּבְקוּמֶךָ".
As far as the answer that the Hasam Sofer gives, it is: Really the verse should continue in the plural, but in order to accentuate the fathers' individual obligation to be examples to their sons with their own consistent learning it spoke about the set times for learning in the singular.