All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we would like to thank you for teaching Muslim children. We ask Allah to reward you for it and add it to your scale of good deeds. As for choosing one of the two books, both of them are important and beneficial. However, Al-Bukhaari's book, Al-Adab Al-Mufrad, is broader in terms of addressing the Islamic manners and etiquette and more relevant to the topic compared to At-Tirmithi's book, Ash-Shamaa'il Al-Muhammadiyyah. Many of the chapters of Ash-Shamaa'il are not directly related to following the example of the Prophet, sallallahu ‘alayhi wa sallam, such as the chapter on the physical description of the Prophet, sallallahu ‘alayhi wa sallam, the one on the description of the Seal of Prophethood (Khaatam An-Nubuwwah), and the one on the heritage of the Prophet, sallallahu ‘alayhi wa sallam.
In any case, both books are useful; however, if you have to choose one of them, we advise you to teach the children Al-Bukhaari's book, Al-Adab Al-Mufrad.
Allah knows best.