Consider using Koiné / Biblical Greek instead of Latin as proposed by Charlotte Mason. Use Modern Greek pronunciation as it is used in Greek Orthodox liturgies instead of reconstructed Erasmian pronunciation.
Biblical Greek provides similar etymological benefits as Latin, especially in science, as over 150,000 English words have Greek roots. For example, σφαῖρα - "spheh-rah" means ball, and it's where the word "sphere" comes from. Greek is particularly helpful with scientific and mathematical vocabulary.
There are some advantages to learning Greek before Latin: children will be decoding words on icons as soon as they start learning the alphabet in Year 4. Latin and Greek have enough grammatical common ground that a child who knows Biblical Greek will find Latin grammatically recognizable. Latin uses the same alphabet as English. And learning Greek or Latin teaches children to recognize root words that are etymologically related to familiar English words. Having that training for Greek will help speed along understanding of Latin.
Biblical Greek is, like Latin, use inflections (changes in ending) rather than word order to indicate what a word is doing in a sentence. Word order in Greek and Latin is significantly freer than in English. Both Biblical Greek and Latin have three grammatical genders (masculine, feminine, and neuter) for nouns, pronouns, and adjectives. Biblical Greek uses copious articles (a, an, the), while Latin doesn't have articles.
St. Raphael Online School offers excellent online classes in Biblical Greek for children and adults.
The most cost-effective method we have found to teach multiple children Biblical Greek when the parents have never learned Greek, is for a parent to take St. Raphael's Adult Biblical Greek classes over two years. This subject is well worth the adult student's time and effort for the depth it adds to reading Scriptures, hymns, and church documents in the original Greek.
Teachers should first acquaint themselves with Charlotte Mason's philosophies on teaching languages, both Latin and French, in her books Home Education (Vol. I) and A Philosophy of Education (Vol. VI), which are available to read for free at AmblesideOnline.org.
The Greek Alphabet Primer by Mallory Stripling at Paidea Classics is a wonderful introduction to the Greek alphabet, its beauty will capture your child's imagination as they learn new letter forms to communicate.
Potamitis Press has an illustrated My Prayer Book in both Greek and English. Purchase the Greek edition to start learning Greek prayers with your child. We recommend starting with Our Father - Πάτερ ἡμῶν.
Memoria Press publishes Elementary Greek I & II. Do not use the CDs to learn pronunciation, as they use reconstructed Erasmian pronunciation, which is not used in Greek Orthodox churches.
Continue learning prayers from My Prayer Book, and practice reading aloud by going to the Greek Orthodox Archdiocese of America's Chapel webpage, selecting the link for the daily Gospel reading, and clicking the Ελληνικἀ link to change the text to Greek from English.
Another Biblical Greek curriculum for children is Hey Andrew, Teach Me Some Greek. These texts move more slowly than the Memoria Press books. Do not use their CDs to learn pronunciation either.
Memoria Press also publishes First and Second Form Greek, which adds in some Attic Greek in with Biblical Greek.
Continue learning Greek prayers, reading the Gospel in Greek, and add in reading aloud and singing church hymns in Greek from the GOA Digital Chant Stand.
Hey Andrew, Teach Me Some Greek addresses Participles and Infinitives in Book 7, and Subjunctive and Imperative Verbs in Book 8.
The Memoria Press Greek textbooks do not address these subjects, and the teacher will have to choose from Hey Andrew, Teach Me Some Greek or using a college Biblical Greek textbook such as Basics of Biblical Greek by Mounce.
Students can use the Interlinear Vespers and Orthros Service Books to continue practicing reading aloud, memorizing, and translating.
Some students may benefit by taking 2-4 years of Latin alongside Biblical/Ancient Greek in High School if they are strongly motivated to read the Classics in the original languages. We recommend finding a Classics Tutor through St. Raphael School, as they have a network of skilled tutors.