I sometimes see posts on the Classical Conversations Facebook page that remind me of an article I should write (gratias tibi ago—thank you). Many times in these posts, people ask when they should start Latin with their child beyond the Latin in the Foundations Guide. It is excellent to start Latin in fourth grade when students begin Essentials; Essentials Latin and English work in tandem. The next logical step is deciding what book to use. Here are my suggestions:
If you like quick and simple, use Getting Started with Latin: Beginning Latin for Homeschoolers and Self-Taught Students of Any Age by William Linney. Each lesson contains a new vocabulary word, pronunciation and/or grammar, and exercises. This book is very straightforward and has a conversational style that I appreciate. The book is self-pacing: students work on one lesson per day, assessments are in the back of the book, and there are extra exercises for students to do if it is too simplistic.
If you want a book that feeds into Henle Latin, use Latina Christiana: Introduction to Christian Latin by Cheryl Lowe. Each lesson has a Latin saying, a few vocabulary words, grammar forms, and then a translation/grammar/derivatives worksheet. The vocabulary and grammar feed into Henle. The pronunciation is ecclesiastical which is different from the classical pronunciation we use in Challenge. The book can be a bit dry, but it is extremely straightforward.
If you want a book that is heavy on grammar and works well with Essentials, use Latin for Children by Christopher Perrin. Each lesson has a Latin saying, ten vocabulary words, grammar, a worksheet, and a quiz. This book is not as straightforward as the others, but it is heavier on grammar and culture. Primer A has a Latin reader companion based on ancient history. Primer B’s reader focuses on medieval and Renaissance history. There are studies on sentence patterns and parsing that go nicely with Essentials work. The one drawback is that the book is less user-friendly to the homeschool mom who has limited Latin experience.
Whatever book you decide to use, use it and stick with it. Do it thoroughly and make sure your student memorizes the vocabulary and grammar in order to know it for his/her entire Latin career. Bona fortuna tecum sit! Good luck to you!