As imam at home, how long should the recitation be so the children don't get bored?

Choose a short, moderate recitation, because a good imam takes account of the people behind him. Short surahs from the last juz' are enough for most prayers, especially if there are young children, a tired wife or an infirm family member. Lengthen it a little now and then, at more relaxed times such as Subuh at the weekend, so that the children get used to new surahs. After the prayer, tell them which surah was recited and invite them to memorise it together that week. A calm, clear recitation leaves a deeper mark than a long, rushed one. If the children start to fidget, that is a sign to shorten the recitation, not to scold. See prayer supplications and recitations.